m 

4-6 


PRIMERS,    edita 
fs  HUXLEY,  ROSCQE,  a 


527   DID 


^^mm   MY- 

J.  N.  LOCKYER. 


Des 


THE  LIBRARY 
OF 

stud 

™t        THE  UNIVERSITY 

can 

»       OF  CALIFORNIA 

obsen 
faculty 
comm 
tion.  PRESENTED  BY 

PROF.  CHARLES  A.  KOFOID  AND 
MRS.  PRUDENCE  W.  KOFOID 


LOCKYER'S  ASTRONOMY. 

ELEMENTS    OF   ASTRONOMY: 

Accompanied  with  numerous   Illustrations,    a  Colored  Repre- 
sentation of  the  Solar,  Stellar,  and  Nebular  Spectra, 
and  Celestial   Charts  of    the   Northern 
and  the  Southern  Hemisphere. 

By  J.  NORMAN  LOCKYER. 

American  edition,  revised  and  specially  adapted  to  the  Schooli 
of  the  United  States. 

izrrtff.    312  pages.    Price,  $1.75. 

The  volume  is  as  practical  as  possible.  To  aid  the  student 
in  identifying  the  stars  and  constellations,  the  fine  Celestial 
Charts  of  Arago,  which  answer  all  the  purposes  of  a  costly  Atlas 
of  the  Heavens,  are  appended  to  the  work — this  being  the  only 
text-book,  as  far  as  the  Publishers  are  aware,  that  possesses  this 
great  advantage.  Directions  are  given  for  finding  the  most  in- 
teresting  objects  in  the  heavens  at  certain  hours  on  different 
evenings  throughout  the  year.  Every  device  is  used  to  make 
the  study  interesting;  and  the  Publishers  feel  assured  that 
teachers  who  once  try  this  book  will  be  unwilling  to  exchange 
It  for  any  other. 

D.  APPLETON  &  CO.,  PUBLISHERS, 

549  &  551  BROADWAY,  NEW  You?:. 


SCIENCE      PRIMERS,    edited  by 

PROFESSORS    HUXLEY,     ROSCOE,     and 

BALFOUR   STEWART. 
ASTRONOMY. 


\ 


A  Lunar  Crater. 


mners. 


ASTRONOMY, 


BY 


J.   NORMAN   LOCKYER,    F.R.S., 
A 

Correspondent  of  the  Institute  of  France, 
Author  of  "Elementary  Lessons  in  Astronomy,"  &*c. 


WITH  ILLUSTRATIONS. 


NEW  YORK: 
D.    APPLETON    AND    COMPANY, 

549  AND  551  BROADWAY. 
1877. 


L(o 

PREFACE. 

IN  writing  this  little  book  I  have  endeavoured 
first  to  help  the  reader,  by  means  of  simple  ex- 
periments, to  form  true  ideas  of  the  motions  of 
the  heavenly  bodies ;  and  then  to  give  a  sketch 
of  the  Earth's  place  in  Nature,  and  of  the  use 
made  of  the  heavenly  bodies  for  Geographical 
purposes. 

I  have  been  much  aided  by  my  friend, 
Mr.  G.  M.  SEABROKE  of  the  Temple  Observatory, 
Rugby,  to  whom  my  acknowledgments  are  due. 

J.  N.  U 

M363157 


CONTENTS. 


INTRODUCTION  f i 

I.  THE  EARTH  AND  ITS  MOTIONS. 

SECT. 

I. — The  Earth  is  round 4 

2. — The  Earth  is  very  large 7 

3. — The  Earth  is  not  at  rest 10 

4, — The  Earth  spins  or  rotates  like  a  top  ...  13 

5. — The  Earth  rotates  once  in  a  day 15 

6. — The  rotation  of  the  Earth  is  not  its  only  motion  19 

7. — The  Earth  travels  round  the  Sun  once  in  a  year  22 
8. — The  two  motions  of  the  Earth  are  not  in  the 

same  plane 23 

9.— Why  the  Days  and  Nights  are  unequal  .  .  ,  26 
10. — The  Seasons  depend  upon  the  difference  in  the 

lengths  of  the  Day  and  Night 33 

1 1.  —  Why  the  movements  of  the  Sun  and  Stars  appear 

different  in  different  parts  of  the  Earth   .     .  35 

II.  THE  MOON  AND  ITS  MOTIONS. 

i. — The  Moon  travels  among  the  Stars    ....  40 

2. — The  Moon  changes  her  form 42 

3.  — How  the  Moon  causes  Eclipses 45 

4. — What  the  Moon  is  like 53 


CONTENTS. 


III.  THE  SOLAR  SYSTEM. 

SECT.  PAGE 

I. — How  bodies  like  the  Earth,  nearer  the  Sun,  would 

appear  to  us 56 

2. — How  bodies  like  the  Earth,  further  off  from  the 

Sun,  would  appear  to  us 58 

3. — Are  there  such  bodies  ? — The  Planets     ...  60 

4. — The  Interior  Planets 62 

5. —The  Exterior  Planets 66 

6.— Comets,  Meteorites,  and  Falling  Stars    ...  77 

IV.  THE  SUN — THE  NEAREST  STAR. 

I. — The  influence  of  the  Sun  in  the  Solar  System  .  81 

2.— The  Heat,  Light,  Size,  and  Distance  of  the  Sun  82 

3.— What  the  Sun  is  like 83 

4. — Sun-spots 84 

5. — The  Sun's  Atmosphere 86 

6. — What  the  Sun  is  made  of 87 

7.— The  Sun  is  the  nearest  Star.     ......  88 

V.  THE  STARS  AND  NEBULA. 

I. — The  Stars  are  distant  Suns 89 

2.— The  brightness  of  the  Stars .89 

3.— The  Constellations 91 

4. — Apparent  movements  of  the  Stars      ....  93 

5  — Real  movements  of  Stars 96 

6.— Multiple  Stars 96 

7.— Clusters  and  Nebulae 97 

8. — The  nature  of  Stars  and  Nebulae loo 


CONTENTS.  xi 


VI.  How  THE  POSITIONS  OF  THE  HEAVENLY  BODIES  ARE 

DETERMINED,  AND  THE  USE  THAT  is  MADE  OF 
THEM. 

SECT.  PAGE 

I. — Recapitulation — Star  Maps  .......  102 

2. — Polar  Distance 103 

3. — Polar  Distance  is  not  sufficient 104 

4. — Right  Ascension 106 

5. — Recapitulation ic8 

6.  —The  Latitude  of  Places  on  the  Earth  .     .     .     .  108 

7. — The  Longitude  of  Places  on  the  Earth    .     .     .  in 

VII.  WHY  THE  MOTIONS  OF  THE  HEAVENLY  BODIES  ARE  so 

REGULAR. 

I.— What  Weight  is 114 

2. — Gravity  Decreased  with  Distance  .     .     .     .     .117 
3. — How  this  explains  the  Moon's  path  round  the 

Earth 118 

4. — The  Attraction  of  Gravitation  .     .     .     .     .     .120 


LIST  OF   ILLUSTRATIONS. 


PAGE 

Plate  I. — Frontispiece.  A  Lunar  Crater  .  .  .  To  face  Title 
,,  2.— The  Solar  System Between  60  &  61 

Fig.  i. — How  ships  become  visible  and  disappear  at  sea  .  4 

,,  2.— Explanatory  of  the  above 5 

,,  3. — Diagram  showing  how,  when  we  suppose  the 
earth  is  round,  we  explain  that  ships  at  sea 

appear  as  they  do 6 

M  4. — Diagram  explaining  how  it  is  that  the  higher  we 

go  the  further  we  can  see 7 

,,  5.—  Diagram  showing  that  the  larger  the  earth  is  sup- 
posed to  be,  the  further  removed  from  us  is  the 
place  at  which  the  sky  appears  to  touch  the 

earth , S 

„  6.— Explanation  of  sun-rise  and  sun-set,  and  star- 
rise  and  star-set 1 1 

,,       7. — The  same 12 

,,       8. — A  top  spinning 14 

,,       9. — The  direction  of  the  earth's  spin 14 

,,     ro. — Experiment  to  illustrate  the  spinning  of  the  earth, 

as  causing  day  and  night 1 5 

,,     ii. — Explanation  of  the  earth's  motion  round  the  sun  19 

,,     12. — The  plane  of  the  ecliptic 24 

,,     13. — Two  planes  cutting  each  other  at  right  angles     .  25 

,,     14. — Two  planes  cutting  each  other  obliquely    ...  25 


LIST  OF  ILLUSTRATIONS. 


PAGB 

Fig.   15. — Earth  with  axis  of  rotation  inclined  to  plane  of 

ecliptic 26 

,,     1 6. — The  Earth,  as  seen  from  the  Sun  at  the  Summer 

Solstice,  June  22  (noon  at  London)   ....     29 

,,     17. — The  Earth,  as  seen  from  the  Sun  at  the  Winter 

Solstice,  Dec.  22  (noon  at  London)   ....     30 

,,     18.— The  Earth,  as  seen  from  the  Sun  at  the  Vernal 

Equinox,  March  22  (noon  at  London)    ...     31 

,,     19. — The  Earth,  as  seen  from  the  Sun  at  the  Autumnal 

Equinox,  Sept.  22  (noon  at  London)      ...     32 

,,     20. — Explanation  of  the  Seasons .     34 

,,  21.— The  Pole  Star  and  the  Constellation  of  the  Great 
Bear  in  four  different  positions,  after  intervals 
of  six  hours,  showing  how  the  Great  Bear 
appears  to  travel  round  the  Pole  Star  ...  36 

,,  22. — The  Moon's  motion  round  the  Earth     ....  43 

.,  23.— Total  Eclipse  of  the  Sun 46 

,,  24. — Annular  Eclipse  of  the  Sun 47 

,,  25. — Eclipse  of  the  Moon 48 

,,     26. — Showing  the  inclination  of  the  Moon's  orbit  to 

the  plane  of  the  ecliptic 50 

,,     27. — Division  of  the  Circle  into  degrees 51 

,,     28. — Diagram  illustrating  the  motions  and  appearances 

of  a  body  between  us  and  the  Sun     ....     57 

9J  29. — Diagram  illustrating  the  motion  of  a  body  travel- 
ling round  the  sun  outside  the  orbit  of  the  earth  59 

,,     30. — Venus,  showing  the  markings  on  its  surface    .     .     64 

,,     31. — Apparent  size  of  Venus,  at  its  least,  mean,  and 

greatest  distance  from  the  Earth 65 

„     32.— Mars,   showing  snow  cap  at  the  pole,   and  the 

lands  and  seas 68 

„     33. — Mars,     View  of  another  part  of  the  planet    .     .     69 


LIST  OF  ILLUSTRATIONS. 


PAGE 

Fig.  34.— Jupiter,  showing  the  cloud  belts 71 

,,     35. — 'Diagram  explaining  the  eclipses,  occultations,  and 

transits  of  Jupiter's  satellites 72 

,,  36. — Saturn  and  his  rings 74 

,,  37. — General  view  of  a  Comet 77 

,,  38. — Head  and  Envelopes  of  a  Comet 79 

,,  39. — How  the  size  of  the  Sun  is  determined       ...  83 

,,  40. — A  Sun-spot * -85 

,,  41. — Explanation   of    the   appearances   presented   by 

Sun-spots 86 

,,  42. — The  Sun's  coronal  atmosphere 87 

,,  43. — Orbit  of  a  Double  Star 97 

,,  44. — The  Cluster  in  Hercules     ...»,,.,,    98 

,,  45. — The  Great  Nebula  in  Orion 99 

.,  46. — How  to  define  the  position  of  anything      .     .     .   104 

„  47. — How  the  positions  of  stars  are  stated    .     .     .     .106 

„     48. — Diagram  showing  the  fall  of  the  Moon  towards 

the  Earth 119 


SCIENCE    PRIMERS. 


ASTRONOMY. 


INTRODUCTION. 

1.  EVERYONE  who   is  going    to    read   this  book 
knows  what  a  school-room  or  school-house  is.     Now 
suppose   it  had   windows   that  you    could    not   see 
through,  and  that  you  never  went  out  of  it :    then 
you  would  think,  perhaps,  that  the  school-house  was  all 
the  world.     But  you  know  better.     You  know  that 
your  school-house  is  only  one  house   out   of  many, 
perhaps  in  the  same  street,  or  at  all  events  in   the 
same  parish,  whether  in  the  country  or  the  town; 
most  of  you  even  will  have  walked  or  ridden  into  the 
Parishes   which   lie  round   the  one  in  which  you 
live. 

2.  If  my  reader  lives  in  London,  he  will  have  done 
more  than  this,  perhaps,  for  if  he  has  crossed  one  of 
the  bridges  over  the  Thames  he  will  have  gone  from 
one  County  to  another  (a  county  being  a  collection 
of  parishes  as  a  street  is  a  collection  of  houses),  for 
the  River  Thames  divides  the  counties  of  Middlesex 
and  Surrey. 

3.  Just  as  a  county  is  a  collection  of  parishes,  so 


SCIENCE  PRIMERS. 


the  Country  of  England,  or  of  Scotland,  or  of 
Ireland,  or  of  Wales,  is  a  collection  of  counties  ;  these 
four  Countries  forming  the  United  Kingdom  of  Great 
Britain  and  Ireland.  Now  wherever  you  are,  whether 
in  a  town  or  village  school,  whether  in  the  United 
Kingdom,  America,  Australia,  or  India,  before  you 
read  the  next  paragraph,  write  down  the 


School, 

Street, 

Parish, 

County, 

Country, 

Kingdom, 


in  which  you  are, 


and  this  will  show  you  that  your  school-house  is  only 
a  very  little  speck  on  the  broad  lands  which  together 
form  the  United  Kingdom,  or  whatever  kingdom  you 
happen  to  be  in. 

4.  Although  you  may  not  have  gone  to  France  or 
Germany,  you  have  heard  of  those  places.     What  are 
they  ?    Well,  the  United  Kingdom,  France,  Germany, 
Russia,  Italy,  Turkey,  and  other  countries,  form  the 
continent  of  Europe,  a  continent  being  a  collection 
of  countries,  as  a  country  is  a  collection  of  counties, 
and  as  a  county  is  a  collection  of  parishes. 

5.  You  may  also   have   heard  of  America,  Asia, 
Africa,  and  Australia,   as  well   as  of  Europe:  nay, 
you  may  even  be  living  in  one  of  these,  which,  like 
Europe,  are  Continents. 

6  Now  these  continents  are  the  largest  stretches 
of  dry  land  on  the  surface  of  The  Earth,  the  surface 
being  partly  water  and  partly  land. 


ASTRONOMY. 


7.  I  have  next  to  tell  you  that  the  earth,  taken 
as  a  whole,  is  a  body  which  astronomers  call 
a  planet :    what  this  is  you  will  learn  by  and  by. 
Before  going  further,  write  down  as  before,  the 

School, 

Street, 

Parish, 

County,      I    .       ... 

Country,     )  ln  whlch  ^  are' 

Kingdom, 

Continent, 

Planet,       / 

8.  Some  of  you  may  think  that  I  have  made  a 
mistake,  and  am  going  to  write  a  book  on  Geography 
instead  of  Astronomy.     I  have  not  made  a  mistake. 
I  want  to  show  you  that  where  Astronomy  leaves  oft 
Geography  begins ;  that  just  as  the  shape,  and  size,  and 
position  of  your  school,  which  is  a  little  speck  on  the 
planet  on  which  we  dwell,  called  the  earth,  can  be 
stated,  and  just  as  men  by  travelling,  can  find  out 
lands  on  the  earth,  far  away  from  your  school,  and  tell 
us  all  about  them,  so  are  the  shape,  size,  and  position 
of  the  earth  itself,  among  all  the  bodies  in  the  skies, 
known,  and  its  relation  to  them  can  be  made  clear  to 
you.     This  is  what  I  have  to  try  to  do,  and  if  I 
can  manage  to  do  it,  then  you  will  understand  better 
when  you  come  to  read  about  the   surface   of  the 
earth. 


SCIENCE  PRIMERS. 


I. -THE  EARTH  AND  ITS  MOTIONS. 
§  I.— THE  EARTH  IS  ROUND. 

9.  Now  I  have  said  that  we  are  on  a  planet  which 
we  call  The  Earth,  but  what  sort  of  thing  is  it?  Is 
it  flat  or  curved,  square  or  round  ?  How  are  we  to 
find  this  out  ?  If  you  look  in  any  direction,  if  you 
are  in  a  hilly  country,  you  see  hills  and  valleys  ;  and 
if  you  walk  over  these  hills,  more  hills  are  generally 
found  rising  up,  which  limit  the  view  to  a  few  miles ; 
if  you  are  in  a  flat  country,  the  trees  and  shrubs 
appear  to  meet  the  sky  in  every  direction  around  you. 
We  may  travel  to  any  place  we  like,  still  there  is  this 
line  where  the  surface  of  the  earth  and  the  sky  meet, 
so  that  for  aught  we  could  tell  to  the  contrary  in  this 


FIG.  i. — How  the  ships  appear  and  disappear  at  sea. 

way,  the  earth  might  be  a  nearly  flat  surface  of  large 
extent. 


ASTRONOMY. 


10.  But  let  us  try  where  there  are  no  rocks  or  trees, 
where  the  surface  of  the  earth  is  unbroken  and  smooth ; 
let  us  try  the  surface  of  the  sea.  Watch  the  ships 
in  the  distance  just  coming  into  view,  and  you  will  find 
that  only  their  masts  are  visible ;  as  they  approach, 
more  and  more  of  the  hull  appears,  until  it  is  quite 
visible.  (Fig.  i).     Now  if  you  watch  a  ship  going  away 
from  you  the  hull  will  disappear  first. 

11.  Now  what  does  this  mean?    Let  us  make  an 
experiment.     Get  a  smooth  table  on  which  there  are 
two  flies,  let  us  say,  and  if  the  flies  are  not  there, 
pretend   that   they  are;    and   suppose   them   to    be 
moving  about.     Now  it  is  clear  that  the  flies,  as  long 
as  they  keep  on  the  surface  of  the  table,  will  always 
be  in  full  view  of  each  other.     They  will  look  smaller 
to  each  other  when  they  are  furthest  apart,  and  larger 


FIG.  2. — Orange  with  flies. 


when  nearer  each  other ;  but  one  part  of  the  fly  will 
not  disappear,  the  other  parts  being  left  visible,  as  in 


SCIENCE  PRIMERS. 


the  case  of  the  ships.     Therefore  the  surface  of  the 
sea  is  not  flat  like  the  surface  of  the  table. 

12.  Another  experiment.     We  will  take  an  orange 
this  time,  and  suppose  a  fly  standing  still  at  the  top, 
say  at  A,  Fig.  2,  and  another  fly  at  the  bottom,  at  B. 
Now  it  is  •  clear  that  the  flies  cannot  see  each  other, 
because  the  orange  is  between  them.     But  suppose  B 
moves  towards  A.    When  it  gets  to  C,  A  can  just  see 
the  top  of  JB's  head  over  the  edge  of  the  orange, 
and  C  can  see  the  top  of  A's  head  over  the  edge. 
No  more  can  be  seen  yet,  because  the  other  parts  of 
each  fly  are  still  hidden  by  the  orange  as  the  whole 
was  before.     But  when  B  gets  still  nearer  to  A,  each 
fly  will  be  in  full  sight  of  the  other. 

13.  We  have  then  by  means  of  the  round  orange 
and  the  moving  flies  managed  to  represent  exactly 
what  happens  on  the  surface  of  the  earth  with  ships, 
though  we  could  not  manage  this  on  the  flat  table. 

14.  Therefore  the  earth  is  like  a  ball  or  an  orange, 
and  not  flat  like  a  table. 

15.  You  will  now  easily  understand  why  we  see  the 
tops  of  ships  first,  and  how  it  is  that  the  higher  we 


FIG  3. — Diagram  showing:  how,  when  we  suppose  the  earth  is  round,  we 
explain  how  it  is  that  ships  at  sea  appear  as  they  do.  At  A  the  ship  is 
invisible,  at  B  its  topmasts  begin  to  be  seen,  and  at  C  it  is  in  full  sight. 

ascend  the  further  we  see.  We  look  over  the 
edge  of  the  earth  in  any  case,  and  the  higher 
we  are  above  the  surface,  the  further  away 
is  the  edge  we  look  over. 


ASTRONOMY. 


1 6.  You  must  not  imagine  from  this  that  there  is 
an  edge  that  you  can  fall  over ;  since  the  earth  is  a 


FlG.  4.— Diagram  explaining  how  it  is  that  the  higher  we  go  the  further 
we  can  see.  To  an  eye  at  A  the  edge  is  at  A  'A ',  to  an  eye  at  B  the 
edge  is  at  B'B' ',  and  so  on. 

globe,  the  apparent  edge  retreats  as  you  advance. 
Think  this  out  for  yourselves  by  help  of  the  orange 
and  flies. 


§  II.— THE  EARTH  IS  VERY  LARGE. 

17.  We  have  employed  an  orange  to  prove  that 
the  earth  is  a  globe.  Some  of  you  may  ask,  "  If  the 
earth  is  round  like  an  orange,  is  it  also  small  like  an 
orange  ?  "  Or  again,  *'  Is  it  fair  to  use  a  smooth  orange, 
while  on  the  earth  there  are  high  mountains  and  all 
manner  of  roughnesses  ?  because,  though  I  can  believe 
that  the  surface  of  the  earth  is  part  of  a  curve  when 
I  look  out  upon  the  sea,  yet  when  I  see  high  moun- 
tains and  deep  valleys,  I  don't  understand  how  such 
an  irregular  surface  can  be  spoken  of  as  part  of  a 
curve."  I  must  try  then  to  answer  these  questions. 


SCIENCE  PRIMERS. 


1 8.  In  the  first  place,  it  is  clear  that  if  you  are 
at  the  same  distance  above  two  globes,  one  large, 
the  other  small,  the  edge  at  which  objects  begin,  or 
cease  to  be,  visible  when  they  are  moving  to  or 
from  the  eye,  will  be  further  off  in  the  case  of  the 
larger  globe. 


FIG.  5  — Diagram  showing  that  the  larger  the  earth  is  supposed  to  be,  the 
further  removed  from  us  is  the  place  at  which  the  sky  appears  to  touch 
the  earth. 


19.  Thus,  in  Fig.  5,  if  A  represent  the  height  of 
the  fly's   eye   above  the  orange  BB,    the    distance 
from  A  to  B  would   represent   the   distance  of  the 
edge  over  which  the  other  fly  would   begin   to   be 
visible,  while  it  would  be  represented  by  the  distance 
from  A  to  C,  if  the  flies  were  on  a  globe  as  much 
larger  than  an  orange,  as  the  circle  indicated  by  CC 
is  larger  than  the  circle  indicated  by  BB. 

20.  Now  since,  when  you  stand  on  the  sea-shore, 
you  can  see  some  miles  out  to  sea,  it  must  be  clear  to 
you  that  the  earth  is  very  large.     This,  then,  answers 
the  first  question.     It  is,  in  fact,  some  8,000  miles  in 
diameter  :  that  is  to  say,  a  straight  line  from  surface  to 
surface  through  the  centre  would  be  8,ooCk  *niles  long. 


ASTRONOMY. 


21.  I  want  next  to  make  you  understand  that  the 
earth,  in  spite  of  its  mountains,  is  really  much  smoother, 
comparatively,  than  an  orange  is. 

Suppose,  for  instance,  that  the  distance  of  the 
surface  of  the  earth  from  the  centre  is  4,000  miles, 
which  is  not  far  from  the  truth.  Then  a  mountain 
four  miles  high  will  only  be  the  one-thousandth  part 
of  this  distance  higher  than  the  general  level,  and 
such  roughnesses  would  be  included  in  the  thickness 
of  the  paper  covering  a  large  school  globe.  You 
will  see  at  once  then  that  the  earth  is  comparatively 
much  smoother  than  an  orange,  for  if  you  were  to 
magnify  an  orange  up  to  the  size  of  a  school  globe, 
it  would  look  very  rough  indeed. 

22.  We  see  then,  (i)  it  is  only  when  the  surface  is 
level,  as  on  a  great  plain  or  on  the  sea,  that  we  can 
judge  by  the  eye  as  to  the  real  form  of  the  earth. 
(2)  But  even  in  the  most  rugged  ground  the  curve  is 
there,  though   we  may  fail  to   notice   it.      (3)  The 
curve,  is  a  very  gentle  one,  because  you  can  see  the 
vessels  at  sea  for  many  miles  before  they  sink  down 
out  of   sight.      (4)   The  facts  that  the  curve  is  so 
gentle,  and  that  the  high  mountains  make  so  little 
difference,  show  that  the  circle  of  which   it  forms  a 
part  is  large,  and  therefore  that  the  earth  itself  is 
large;   and   (5)    the  earth  is  so  big,  that  even  the 
highest  mountains  are  in  comparison  merely  like  little 
grains  on  the  surface ;  its  diameter  or  distance  from 
side  to  side  through  its  centre  is  8,000  miles. 


10  SCIENCE  PRIMERS. 


§  III.— THE  EARTH  IS  NOT  AT  REST. 

23.  The  Earth,  then,  with  its  surface  of  land  and 
water,  is  a  great  globe,  so  big  that  supposing  there 
were  a  road  all  round  it  from  your  school,  and  that 
you  were  to  walk  on  day  and  night  without  rest,  at 
the  rate  of  three  miles  an  hour,  it  would  take  you 
nearly  a  year  to  get  to  school  again. 

24.  The  earth,  too,  hangs  in  space  as  you  some- 
times see  a  balloon.     Now  is  it  at  rest?  or  does  it 
move  ?    Perhaps  you  will  say  that  it  does  not  move, 
because   your   school-house  is  where  it  always  was ; 
that   the  houses  or  trees  near  to  it   are  no  further 
away  or  nearer  than  they  were. 

25.  But  this  does  not  help  us  :  let  us  take  a  large 
ball  of  worsted,  or  an  orange,  to  represent  the  earth, 
and   stick   into  it  one  pin  to  represent  the   school- 
house,  and  other  pins  to  picture  to  you  the  trees  and 
homes  round  it. 

26.  You  will  see  at  once  that  whether  the  worsted 
ball  or  the  orange  is  at  rest  or  in  motion,  the  positions 
of  the  pins  with  regard  to  each  other  will  not  change. 

27.  How,  then,  are  we  to  settle  the  question?    By 
looking  at  something  not  on  the  earth.  Go  out 
on  a  clear  evening,  and  look  in  the  east  (every  boy 
and  girl  should  know  where  the  north,  south,  east,  and 
west  points  are) :  you  will  see  the  stars  rising  higher 
and  higher  above  the  edge  of  the  earth,  that  is,  the 
line  where  the  earth's  surface  and  the  sky  meet,  which 
we  must  henceforth  call   the   horizon.     Those  in 
the  west  will  be  gradually  disappearing  just  in  the 
same  way ;  the  moon  also  follows  their  example.     In 


ASTRONOMY.  11 


the  day-time  we  find  that  the  sun  rises  in  the  east 
and  sets  in  the  west,  in  exactly  the  same  manner. 

28.  Here  there  is  proof  positive  that  while   the 
houses  and  trees  on  the  earth's  surface  do  not  move 
with  regard  to  each  other,  the  sun,  stars,  and  moon, 
which  are  not  on  the  earth's  surface,  do  move,  or 
appear  to  move,  with  regard  to  the  earth. 

29.  Now  let  us  think  about  this.    What  do  we  mean 
when  we  say  that  a  star  or  the  sun  rises  and  sets  ? 
We  mean  that  it  is  just  passing  either  up  or  down  over 
the  edge  of  the  earth  seen  from  the  place  where  we 
are ;  the  sun  or  star  in  fact  does,  or  appears  to  do,, 
just  what  the  ships  that  we  referred  to  in  par.  10  did.. 
The  ball  of  worsted  or  the  orange  should  make  this 


FIG.  6. — Explanation  of  sun-rise  and  sun-set,  and  star-rise  and  star-set. 

quite  clear.  Put  it  on  the  middle  of  a  table,  and 
stick  a  pin  into  its  side,  the  pin's  head  to  represent 
your  eye.  Now  imagine  yourself  to  be  the  sun  or 
a  star,  and  walk  round  the  table  as  represented  in 
Fig.  6,  keeping  your  eye  on  a  level  with  the  pin ;  at 
one  point  the  pin  will  be  seen  just  rising  from  the 
edge  of  the  ball ;  you  are  playing  the  part  of  a  rising 
sun  or  s'tar,  to  your  own  eye  represented  by  the  pin's 


12  SCIENCE  PRIMERS.  [§  HI. 

head;  at  another  point  in  your  journey  round  the 
table  the  pin's  head  will  disappear,  and  at  last  will  be 
hidden  by  the  edge  of  the  ball.  Here  you  are  playing 
the  part  of  a  setting  sun  or  star,  supposing  the  earth 
to  be  at  rest. 

30.  Now  sit  down  and  get  someone  to  turn  the  ball 
of  worsted  round  for  you,  keeping  the  pin's  head 
always  at  the  same  distance  above  the  table.  In  this 
case,  the  motion  of  the  ball,  while  you  are  at  rest,  will 
give  rise  to  the  same  appearances  as  those  you  saw 
when  the  ball  was  at  rest,  and  you  walked  round  it. 


Fig.  7. — Diagram  explaining  Fig.  6  ;  with  the  direction  of  motion  indicated 
a  body  at  A  is  setting,  at  B  is  rising,  and  at  C  is  overhead. 

31.  Hence  the  appearances  connected  with  the 
rising  and  setting  of  the  sun  and  stars,  may  be  due 
either  to  our  earth  being  at  rest  and  the  sun  and 
stars  travelling  round  it,  or  the  earth  itself  turning 
round,  while  the  sun  and  stars  are  at  rest.  The 
ancients  thought  that  the  earth  was  at  rest,  and  that 
the  sun  and  stars  travelled  round  it.  But  we  now 
know  that  it  is  the  earth  which  moves. 


ASTRONOMY.  13 


§  IV.— THE    EARTH  SPINS  OR   ROTATES  LIKE 
A  TOP. 

32.  You  have  then  to  take  it  as  proved  that  the  earth 
moves,  and  that  the  seeming  movements  of  the  sun, 
moon,  and  stars,  as  they  travel  from  east  to  west,  the 
sun  by  day,  and  the  moon  and  stars  by  night,  are  not 
real  movements,  but  are  apparent  movements  only, 
brought  about  by  the  actual  movement  of  the  earth. 

33.  How  then  does  the  round  earth  move?    Let 
us  think  a  little.     Have  we  any  familiar  example  01 
such  apparent  movement  of  objects  at  rest  brought 
about  by  our   own   movement?     Yes,    certainly  we 
have.     You  will  all  at  once  think  how,  when  you 
are  sitting  in  a  railway-carriage,  all  the  objects,  trees, 
houses   and  what  not,  that  you  can  see  out  of  the 
window  and  are  really  at  rest,  appear  to  fly  past  you  as 
if  you  were  at  rest.     Further,  they  appear  to  sweep 
past  you  in  the  direction  exactly  opposite  to  the  one 
in  which  you  are  going. 

34.  So  far  so  good.     Now  will  it  do  to  apply  this 
reasoning  at  once  to  the  earth  and  stars,  to  imagine 
that  the  whole  earth  is  really  moving  rapidly  from  the 
point  that  we   call  West   towards   the  East,   and  is 
rushing  rapidly  past   the  sun   and  moon  and  stars? 
and  that  this  is  the  reason  they  appear  to  move  from 
East  to  West? 

35.  You  will  at  once  see  that  it  will  not  do  to  reason 
thus,  because  we  should  thus  never  see  the  same  sun 
and  moon  and  stars  again. 

36.  How  then  can  we  explain  the  facts  ?    We  can 
imagine  that  the  earth  spins  round  as  a  top 


SCIENCE  PRIMERS. 


[§iv. 


does,   so  that  every  morning   every  boy  and  girl, 
whether  living  in  England,  or  America,  or  Australia, 


FIG.  8.— A  top  spinning. 

or  India,  sees  the  same  sun  rise,  and  every  evening 
sees  the  same  sun  set. 

37.  It  is  in  fact  because  the  earth  does  turn  in  this 
,way  that  we  have  morning  and  evening  at  all,  and  day 


FIG.  9. — The  direction  of  the  earth's  spin. 

and  night  are  the  best  proofs  that  the  earth  does 
really  spin  as  I  state  that  it  does. 


ASTRONOMY.  15 


38.  And  because  the  sun  seems  to  rise  in  the  East 
and  set  in  the  West,   the  earth  really  spins   in  the 
opposite  direction,  that  is,  from  West  to  East. 

39.  Now  get  a  common  school  globe.     Set  it  spin- 
ning as  you  would  a  top ;   that  is,  let  the  axis  be 
upright  as   a   top's   is.     Which  way   is   it   to   turn? 
With  your  right  hand  push  the  right-hand  surface  of 
the  globe  away  from  you.     The  globe  then  represents 
the  direction  in  which  the  real  earth  turns. 

§  V.— THE  EARTH  ROTATES  ONCE  IN  A  DAY. 

40.  Take  an  orange,  to  represent  the  earth,  into  a 
dark  room,   with  a  lamp  to  represent  the  sun;  stick 
a  knitting  needle  through  the  centre  of  the  orange, 
and  then  upright  into  a  pincushion  having  also  stuck 
a  small  pin  as  far  as  it  will  go  into  the  orange,  so  that 


FIG.  10. — Experiment  to  illustrate  the  spinning  of  the  earth,  as  causing  day 
and  night. 

its  head  shall  represent  an  observer  on  the   earth. 
Twist  the  needle  round,  and  so  make  the  orange  turn 


16  SCIENCE  PRIMERS.  [§v. 

round  slowly,  in  the  contrary  direction    to   that   in 
which  the  hands  of  a. watch  move,  as  in  Fig.  9.    • 

41.  Examine  what  happens.    First,  there  will  be  two 
points  on  the  orange  through  which  the  knitting  needle 
passes,  which  do  not  move,  and  these  are  called  the 
poles,  the  one  at  the  top  we  will  call  the  north 
pole,  and  the  bottom  one  the  south  pole,  the  line 
joining  the  poles  we  will  call  the  axis  ;  this  is  repre- 
sented by  our  needle.    Draw  a  circle  round  the  middle 
of  the  orange,  everywhere  at  the  same  distance  from 
the  poles,  or  just  where  we  should  cut  the  peel  if 
we  were  going  to  cut  a  lily  or  other  similar  device 
from  the  fruit:  this  line  we  will  call  the  equator. 
Let  the  pin's  head  be  near  this  line  and  opposite  the 
lamp  representing  the  sun.     One  half  of  the  .orange 
will,  of  course,  be  lighted  up  by  the  lamp,  representing 
day,  and  the  other  half  dark,  representing  night. 

42.  Now  twist  the  knitting  needle  slowly,  and  you 
will  see  that  the  pin's  head,  instead  of  being  exactly 
in  the  middle  of  the  half  of  the  orange  first  lit  up  ly 
the  lamp,  will,  when  the  orange  has  turned  through  a 
quarter  of  a  circle,  be  just  visible  at  the  edge  of  the 
lighted  portion ;  a  slight  turn  more,  and  no  light  reaches 
it, — the  lamp  has  set.     Turn  the  orange  another 
quarter  of  a  circle,  and  you  find  the  pin's  head  is 
in  the  centre  of  the  dark  side,  with  its  head  turned 
exactly  opposite  to  the  lamp ;  another  quarter's  turn, 
and  the  pin's   head  is  just  coming  into   the   lamp- 
light— the  lamp   is    rising ;    a  quarter  of  a  turn 
more,  and  the  orange  has  turned  round  once,  and  the 
lamp  is  again  shining  directly  overhead  as  at  first. 

43.  The  lamp  has  therefore  apparently  passed  from 
over  the  pin's  head,  set,  and  risen,  and  come  to  the 


ASTRONOMY.  17 


same  place    again,   simply  by  turning    the    orange 
round. 

44.  So  with  the  earth,  it  rotates  as  the  orange  has 
done,  in  the  same  way,  round,  not  a  knitting-needle, 
but  an  imaginary  axis,  passing  through  its  poles. 

45.  Day  and  night  are  thus  caused,  and  as   the 
sun  appears  to  take  twenty-four  hours  to  move  from 
where  it  is  at  any  time  to  the  same  place  again  the 
next  day,  we  learn  that  the  earth  actually  takes  twenty- 
four  hours  to  turn  once  on  its  axis.     (Par.  41.) 

46.  It  is  time  now  that  we  made  use  again  of  an 
ordinary  school-globe.    Get  one  of  these  and  place  the 
lamp  a  few  feet  from  it,  on  a  level  with  its  centre.    Let 
the  axis  of  the  globe  be  upright,  and  make  the  globe 
turn  round.     Whether  it  is  allowed  to  remain  at  rest 
or  is  sent  spinning  round  rapidly,  the  half  of  it  next 
the  lamp  will  be  illuminated,  and  the  other  half  away 
from  the  lamp  will  be  in  shade.     When  it  is  at  rest, 
the  places  on  one   side  remain  in  the   light,  while 
those  on  the  opposite  side  remain  in  the  dark.     As 
you  turn  it  round,  each  place  in  succession  is  brought 
round  to  the  light,    and  carried  on   into   the  shade 
again.     And  while  the  lamp  remains  unmoved,  the 
rotation  of  the  globe  brings  alternate  light  and  dark- 
ness to  each  part  of  its  surface. 

47.  Now,  instead  of  the  little  school-globe,  imagine 
the  earth,  and  in  place  of  the  feeble  lamp,  the  great 
sun,  and  you  will  see  how  the  rotation  or  spinning 
round  of  the  earth  on  its  axis  must  bring  alternate 
light  and  darkness  to  every  country. 

48.  You  must  not  suppose  that  there  is  any  actual 
rod  passing  through  the  earth  to  represent  our  knitting- 
needle  and  the  steel  rod  of  the  school-globe,  to  form 


1 8  SCIENCE  PRIMERS.  [§  v. 

the  axis  round  which  it  turns.  The  axis  is  only  an 
imaginary  line,  and  the  two  opposite  points  where  it 
reaches  the  surface,  and  where  the  ends  of  the  rod 
would  come  out  were  the  axis  an  actual  visible  thing, 
are  still  called  the  North  Pole  and  the  South 
Pole,  both  on  the  globe  and  on  the  earth  itself. 

49.  The  earth  spins  then  round  this  axis  once  in 
every  twenty-four  hours.     All  this  time  the   sun   is 
shining  steadily  and  fixedly  in  the  sky.    But  only  those 
parts  of  the  earth  can  catch  his  light  which  happen  at 
any  moment  to  be  on  the  side  turned  towards  him. 
There  must  always  be  a  bright  side  and  a  dark  side, 
just  as  there  was  a  bright  side  and  a  dark  side  when 
you  placed  first  the  orange  and  then  the  globe  oppo- 
site to  the  lamp.    Now  you  can  easily  see  that  if  there 
were  no  motion  in  the  earth,  half  of  its  surface  would 
never  see  the  light  at  all,  while  the  other  half  would 
never  be  in   darkness.     But  since  it  rotates,  every 
part  is  alternately  in  sunlight  and  in  darkness.     When 
we  are  catching  the  sun's  light,  we  have  Day  ;  when 
we  are  on  the  dark  side,  we  have  Night. 

50.  The  sun  seems  to  move  from  east  to  west.    The 
real  movement  of  the  earth,  is,  for  a  reason  which 
has  been  stated  in  par.  38,  just  the  reverse  of  this, 
viz.   from  west   to   east.     In   the    morning   we   are 
carried   round   into   the   sunlight,  which  appears   in 
th'e  east.     Gradually  the  sun  seems  to  climb  the  sky 
until  he  appears  highest  at  noon,  and  gradually  he 
sinks  again  to  set  in  the  west,  as  the  earth  in  its 
rotation  carries  us  round  once  more  out  of  the  light. 
At  night  we  trace  the  movement  of  the  earth  by  .the 
way  in  which  the  stars  one  by  one  rise  and  set,  as 
the  sun  rises  and  sets  in  the  daytime. 


ASTRONOMY. 


VI.— THE  ROTATION  OF  THE  EARTH  IS  NOT 
ITS  ONLY  MOTION. 

51.  You  are  now  probably  convinced  of  these  facts. 
First,  that  the  earth  is  a  globe. 
Secondly,  that  the  earth  spins  like  a  top. 


WALL   A 


TABLE 


WALL    C 


FIG.  ii.— Explanation  of  the  Earth's  motion  round  the  Sun. 

And  lastly,  that  without  this  spinning  there  could 
be  no  day  and  night,  so  that  the  regular  succession 
of  day  and  night  is  caused  by  this  spinning. 


20  SCIENCE  PRIMERS.  [§  vi. 

52.  Here  then  we  have  fairly  proved  that  the  earth 
has  one  motion.     Now  the  question  comes,  has  it 
more  than  one?     How  shall  we  settle  this?     Well, 
first  of  all  let  us  see  if  this  one  motion  will  account 
for  all  the  things  we  see. 

53.  To  do  this  we  must  again  have  our  globe  and 
orange,  and  imagine  them  in  a  room  with  many  pic- 
tures on  the  walls.     You  wonder  what  pictures  have 
to  do  with  it  ?     Well,  I  want  the  pictures  to  represent 
the  stars  in  the  sky.     There  are  stars  all  round  the 
part  of  space  in  which  the  earth  and  the  sun  are,  only 
we  cannot  see  them  in  the  daytime,  because  the  sun 
is  so  bright.     So  that  if  you  have  pictures  all  round 
the  globe  and  orange  they  will  represent  the  stars. 
Of  course  there  should  be  pictures  on  the  ceiling 
and  floor   too,    but    we   will    content   ourselves  by 
imagining  them  to  be  there  as  well. 

54.  Now   imagine    the    globe    at    rest    and    the 
orange     at    rest.       Do    not    turn    it    round    even. 
Then,    as    we    have    already   seen,   if  we    imagine 
the  orange    to   represent    the  earth,  and   the   lamp 
to  represent  the  sun,  that  part  of  the  orange  turned 
to  the  sun,  represented  by  the  lamp,  will  have  per- 

petual day,  and  will  always  see  the  same  \  gun  \ 
in  the  same  place  ;  from  that  part  of  it  turned  away 
from  the  sun  the  same  |  ^r™  j  will  always  be 
visible  in  the  same  place.  From  the  parts  of  the 

near  the  boundarv  of  lls^  and  shade 


***»>*    features}    will  be  for  ever  ap- 


ASTRONOMY. 


parently   near   the   horizon   (par.    27)   in   the   same 
place. 

55.  Now  stick  a  pin  in  the  equator  (par.  41)  of  the 
orange  up  to  the  head,  to  represent  an  observer  on  the 
earth,  turn  the  orange  round  to  represent  the  spinning 
or  rotation,  as  we  must  now  call  it,  of  the  earth,  and 
mark  that  whenever  the  observer  represented  by  the 
pin's  head  is  in  the  middle  of  the  lighted-up  half,  the 
part  exactly  opposite  is  in  the  middle  of  the  dark 
half,  and  that  half  a  turn  of  the  orange  brings  the 
pin's  head  from  the  middle  of  the  lighted-up  to  the 
middle  of  the  dark  portion.    Now  these  two  positions 
— namely,  the  middle  of  the  lighted-up  half  and  the 
middle  of  the  dark  half — represent  nearly  enough  for 
our  present  purpose  the  position  with  regard  to  the 
sun  which  an  observer  is  made  to  occupy  at  midday 
and  midnight  by  the  earth's  rotation. 

56.  You  will  see  in  a  moment,  therefore,  that  if 
neither  the  sun  nor  the  earth  move  from  their  places, 
we  shall  always  see  one  particular  set  of  stars  at  mid- 
night, another  particular  set  at  sunrise,  and  another 
particular  set  at  sunset. 

57.  Think  this  well  over  and  reason  it  out  with  the 
pictures,  for  it  is  a  very  important  point  for  you  to 
understand  clearly. 

58.  Now,  is  it  a  fact  that  we  always  do  see  the 
same  stars  at  midnight?     No.     Then  what  are  the 
facts  ? 

(i).  If  we  view  the  stars  at  midnight  in  summer, 
and  again  at  the  same  time  in  winter,  we  see 
different  stars.  Here  then  is  a  great  change  in  six 
months. 

(2).  If  we  view  the  stars  for  many  nights  in  succes- 


22  SCIENCE  PRIMERS.  [§  vn. 

sion  at  midnight,  we  find  them  gradually  falling  away 
to  the  west.     Here  is  a  slight  change  in  a  few  days. 

(3).  After  the  lapse  of  a  year  the  same  stars  are 
visible  at  midnight. 

59.  Now  move  the  orange  round  the  lamp  in 
the  same  direction  as  the  earth  rotates,  and 
you  will  see  at  once  that  this  explains  all  the  facts. 

60.  In  Fig.  n,  I  have  given  a  drawing  of  the  lamp, 
orange,  table,  and  room,  as  you  would  see  them  from 
above.     First  consider  the  orange  at  A.     Then  at  mid- 
night the  observer  on  the  dark  side  would  see  the  stars 
opposite  to  the  sun,  the  pictures  on  wall  A :  at  B,  at 
midnight  he  would  see  the  stars  opposite  the  sun,  now 
represented  by  the  pictures  on  wall  B ;  and  therefore 
no  longer  the  same  stars  as  were  seen  before.     So 
on  with  the  positions  at  C  and  D. 

6 1.  I  must  next  point  out  to  you  that  the  same  effects 
would  be  produced  as  those  we  see  and  have  thus 
accounted  for,  by  supposing  the  sun  to  travel  round 
the  earth  in  the  opposite  direction.     But  we   know 
that  the  earth  really  travels  round  the  sun,  and  not 
the  sun  round  the  earth. 


§  VII.— THE  EARTH  TRAVELS  ROUND  THE 
SUN  ONCE  IN  A  YEAR. 

62.  The  earth  then  not  only  rotates  on  its  axis 
once  a  day,  but  travels  round  the  sun.  In  this  way  we 
have  accounted  for  the  fact  that  as  seen  at  midnight,  or 
at  the  same  hour  every  night  from  any  part  of  the 
earth,  whether  England,  America,  Australia,  or  India, 
the  stars  visible  are  continually  changing.  We  have 
found  also  that  they  change  very  little  in  a  few 


ASTRONOMY.  23 


nights,  very  much  in  six  months,  and  that  after 
twelve  months  the  same  stars  again  appear  in  the 
same  places. 

63.  Now  my  reader  should  again  go  to  his  lamp 
and  orange,  and  he  will  find   that  precisely  as  the 
earth  spins  in  a  day,  so  it  goes  round  the  sun 
in  a  year. 

64.  For  it  is  clear  that  if  for  instance  the  journey 
only  required  six  months,  then  in  six  months  the  same 
stars  would  be  visible  at  midnight,  and  so  on  for  any 
other   period   you    might  choose  to  suggest.     Here 
then  we  have  the  origin  of  the  year,  which  is  the  time 
the  earth  requires  to  get  back  to  the  same  place  in  its 
path  round  the  sun. 


§  VIII.— THE  TWO  MOTIONS  OF  THE  EARTH 
ARE  NOT  IN  THE  SAME  PLANE. 

65.  "  How  does  the  earth  travel  round  the  sun  ? 
does  it  jerk,  or  go  up  and  down,  or  always  smoothly 
and  right  on,  keeping  the  same  level  ?  "  some  of  you 
may  ask.     I  answer,  the  earth  travels  smoothly,  and 
always  keeps  the  same  level ;  as  horses  do.  galloping 
round  a  very  level  race-course.     To  picture  this  more 
exactly,  imagine  a  very  large  ocean  with  the  sun  and 
earth  floating  on  it  up  to  their  middles,  then  imagine 
the  earth  to  travel  thus  round  the  sun  once  a  year 
in   a   nearly   circular  path,  that   is,  always   keeping 
about  the  same  distance  from  the  sun. 

66.  Now  get  five  balls,  one  larger  than  the  others, 
to  represent  the  sun  ;  weight  them  so  that  they  sink 
up  to  their  middles,  and  then  put  them  in  a  tub  of 
water  as  shown  in  Fig.  12. 

4 


24  SCIENCE  PRIMERS.  [§  vin. 

67.  We  have  now  a  representation  of  the  sun,  and  of 
the  earth  in  four  parts  of  its  annual  journey.  What 
I  want  you  to  understand  is  that  the  motion  of  the 
earth  is  not  only  smooth,  but  that  its  motion  is  in 
the  same  plane,  a  plane  being  a  level  surface  re- 
presented by  a  sheet  of  cardboard  or  the  surface  of 
the  water  in  the  tub  :  and  next  that  this  plane  in 


Fig.  12.— The  plane  of  the  Ecliptic. 

which  the  earth  moves  passes  through  the  centres  of 
the  sun  and  earth,  as  the  centres  of  the  balls  will  be 
on  a  level  with  the  water  if  you  have  weighted  them 
properly.  Further  let  me  call  the  plane  represented 
by  the  level  surface  of  the  water  the  Plane  of  the 
Ecliptic. 

68.  Here  then  is  defined  the  plane  of  the  earth's 
motion   yearly   round    the    sun ;    this    plane    of   the 
ecliptic  is  the  earth's  race-course.     What  is  the  rela- 
tionship  of   this   to   the   plane   of  the   earth's   daily 
motion  round  its  axis  ? 

69.  Now  it  is  clear  that  if  the  earth's  axis  is  sup- 
posed to  be  upright  with  regard  to  the  plane  of  the 


ASTRONOMY. 


ecliptic,  or  to  form  a  "  right  angle  "  with  it,  the  plane 
of  the  earth's  spin  will  be  the  same  as  the  plane 
of  the  earth's  motion  round  the  sun.  This  is  the 
state  of  things  represented  in  Fig.  12. 

70.  But  are  these  planes  the  same?  Let  us  sup- 
pose them  to  be  so.  Stick  a  pin  into  one  of  the 
smaller  balls,  make  the  ball  spin  uprightly  like  a  hum- 
ming top,  and  it  will  represent  the  earth  as  it  travels 
round  the  sun,  and  you  will  find  that  on  this  sup- 


FIG.  13.— Two  planes  cutting-  each  other  at  right  angles. 

position,  the  days  will  always  be  of  the  same  length, 
because  the  boundary  of  light  and  darkness  would 


FIG.  14.— Two  planes  cutting  each  other  obliquely. 

pass  through  the  two  poles,  so  that  each  part  of  the 
earth's  surface  would  be  an  equal  time  in  the  lighted 


SCIENCE  PRIMERS. 


up,  and  in  the  dark,  half,  if  the  motion  of  rotation 
were  uniform.  But  the  days  are  not  all  of  the  same 
length  ;  in  winter  in  England  they  are  short,  and 
the  nights  are  long;  and  in  summer  the  days  are 
long,  and  the  nights  are  short ;  and,  further,  while  it 
is  Christmas  here  in  England  and  America  it  is 
summer  in  Australia. 

71.  So  then  the  planes  of  the  two  motions  cannot 
be  coincident ;  but  we  can  explain  all  the  facts  by 
assuming  them  to  be  inclined  to  each  other  as  shown 
in  Fig.  14,  so  that  the  earth's  axis  in  its  journey  round 
the  sun  is  really  represented  by  the  little  balls  in 
Fig.  15,  in  which  they  no  longer  spin  upright  as  in 
Fig.  12,  but  their  axes  are  inclined. 


FIG.  15. — Earth  with  inclined  axis  of  rotation. 

5  IX.— WHY   THE    DAYS  AND  NIGHTS  ARE 
UNEQUAL. 

72.  We  can  now  leave  the  tub,  and  come  back  to 
the  lamp  and  orange,  remembering  that  the  knitting- 
needle  must  no  longer  be  upright  as  we  allowed  it 


ASTRONOMY.  27 


to  be  in  Fig.  10,  and  that  the  plane  of  the  ecliptic  is 
represented  by  the  horizontal  plane  in  which  lies  the 
line  joining  the  centre  of  the  lamp  and  the  centre  of 
the  orange. 

73.  We  have  before  accounted  for  day  and  night, 
now  let  us  see  if  we  can  explain  why  they  differ  in 
length,  at  different  seasons  of  the  year.     Place  the 
lamp  as  before  on  a  table  in  the  middle  of  the  room, 
and  support  the  orange  at  the  same  height  as  before, 
inclining  the  upper  end  of  the  knitting-needle 
a  little  way  from  the  lamp.     Let  us  call  the 
upper  pole  the  north  pole. 

74.  Now  turn  the  orange  round,  and  you  will  see 
that  the  light  never  shines  on  the  part  of  the  orange 
near  the  north  pole,  and  always  shines  on  a  part  round 
the  south  pole,  however  rapidly  you  turn  the  orange ; 
but  that,  as  before,  parts  near  the  equator  alternately 
become  lighted  and  darkened.     Now  stick  a  pin  in 
the  orange,  to  represent  an  observer  near  the  north 
pole,  and  again  twist  the  orange,  and  you  will  see  that 
he  never  gets  into  the  light  region ;  stick  it  near  the 
south  pole,  and  here  he  will  always  see  the  lamp,  so 
that,  with  the  earth  in  this  position  with  regard  to  the 
sun,  to  a  person  at  the  north  pole  it  is  always  night, 
and  at  the  other  pole  always  day. 

75.  Again  stick  the  pin  in  the  orange,  about  half-way 
between  the  equator  and  the  north  pole,  and  twist  the 
orange,  and  you  will  see  that,  as  it  travels  round  with 
the  orange,  it  has  a  much  longer  journey  round  on  the 
dark  side  of  the  orange  than  it  has  on  the  light  side. 
At  this  point,  therefore,  the  night  is  much  longer  than 
the  day,  and  you  will  see  that  the  nearer  you  place 
the  pin  to  the  north  pole,  the  shorter  will  be  its  period 


28  SCIENCE  PRIMERS.  [§  ix. 

of  illumination,  till  it  gets  so  far  north  as  never  to  be 
illuminated  at  all. 

76.  On  the  other  hand,  the  nearer  you  place  the 
pin  to  the  equator  in  the  northern  half  of  the  orange 
the  longer  it  is  lighted,  or  the  days  become  longer 
and  the  nights  shorter,  till  on  the  equator  the  journey 
in  the  light  is  just  equal  to  that  in  the  dark. 

77.  Exactly  the  reverse  takes  place  on  the  south 
side  of  the  equator;  the  further  you  place  the   pin 
towards  the  south  pole,  the  longer  will  its  journeys  in 
the  light  become,  till  near  the  pole  it  never  passes 
into  darkness. 

78.  Now  if  you  increase  the  inclination  of  the  knit- 
ting-needle away  from  the  lamp,  you  will  see  that  the 
days  and  nights  become  more  and  more  unequal  at  any 
place  where  you  choose  to  place  the  pin,  except  at  the 
equator,  and  the  less  you  incline  it  from  the  lamp 
the  less  is  the  inequality,  so  that  when  it  is  upright, 
day  and  night  are  equal  all  over  the  orange.     Now 
you  all  know  that  England  is  on  the  north  side  of 
the  equator,  about  half-way  between  the  equator  and 
pole,  but  somewhat  nearer  the  pole  than  the  equator ; 
and  you  also  know  that  in  winter  the  days  are  much 
shorter  than  the   nights,  and  we  at   once  therefore 
account  for  this  by  supposing  the  axis  of  the  earth  to 
be  tipped  in  the  same  manner  and  direction  as  that  of 
the  orange,  so  that  the  orange  in  the  case  just  men- 
tioned represents  the  earth  in  the  winter. 

79.  It  is,  however,  not  always  winter  with  us,  and 
following  winter  comes  spring,  when  the  days  and 
nights  are  equal  in  length  on  March  22  ;  then  comes 
summer  in  three  months  more,  when  the  days  are 
longer  than  the  nights ;  just  the  reverse  of  what  hap- 


ASTRONOMY.  29 


pens  in  winter.  In  autumn,  on  September  22,  the 
days  and  nights  are  again  equal.  How  can  we  ac- 
count for  this?  Let  us  consider,  and  return  to  our 
orange;  we  might  try  to  explain  it,  by  tipping  the 
orange  less  and  less  till  the  axis  is  upright  to  re- 
present spring,  and  then  tip  it  towards  the  lamp  to 


FIG.  16. — The  Farth,  as  seen  from  the  Sun  at  the  Summer  Solstice, 
June  22  (noon  at  London). 


represent  summer,  for  you  will  see  from  what  has 
been  said  before,  that  if  the  north  pole  be  turned 
away  from  the  lamp,  the  nights  are  longer  than  the 
days ;  when  it  is  upright  they  are  equal ;  and  when 
it  is  turned  towards  the  lamp,  the  days  are  longer 


SCIENCE  PRIMERS. 


[§«. 


than  the  nights ;  but  the  earth's  axis  does  not  alter 
in  its  direction,  as  we  always  find  that  the  axis  points 
very  nearly  to  the  same  star,  called  the  pole-star,  at 
all  times  of  the  year. 

80.  We  must  therefore  try  another  method.     Move 


FIG.  17.— The  Earth,  as  seen  from  the  Sun  at  the  Winter  Solstice, 
Dec.  22  (noon  at  London). 


the  orange  the  contrary  way  to  the  hands  of  a  watch, 
round  the  lamp,  still  keeping  the  axis  pointing  in  the 
same  direction,  or  more  correctly,  keeping  the  axis 
represented  by  the  knitting-needle  always  parallel  to 
itself;  let  it  be  moved  a  quarter  of  the  way  round  the 
lamp  and  rotate  the  orange,  and  observe  the  length 
of  day  and  night  as  before ;  you  will  see  that  the 


ASTRONOMY. 


poles  are  on  the  boundary  which  separates  the  light 
from  the  dark  half,  and  the  journey  of  every  part  of 
the  orange  through  light  and  darkness  is  equal. 
This  position  corresponds  to  the  commencement  of 
spring,  March  22. 

8r.  Move  the  orange  another  quarter  of  a  circle 


FIG.  1 8.— The  Earth,  as  seen  from  the  Sun  at  the  Vernal  Equinox, 
March  22  (noon  at  London). 

round  the  lamp  ;  now  you  see  the  north  pole  is  tilted 
towards  the  lamp,  and  at  every  place  north  of  the 
equator,  or  in  the  northern  half,  or  hemisphere,  day 
is  longer  than  night,  corresponding  to  summer,  and 
the  reverse  at  the  southern  hemisphere,  so  we  have 
matters  just  reversed  by  moving  the  orange  half- 
way round  the  lamp. 


32  SCIENCE  PRIMERS.  [§  ix. 

82.  Another  quarter's  turn,  and  day  and  night  are 
again    equal,    corresponding   to   autumn,    Sept.    22 ; 
one  more  quarter  brings  the  orange  to  its  original 
position. 

83.  Just  in  the  same  way  the  earth  moves  round  the 
sun  in  a  year,  passing  from  winter  through  spring  to 


FIG.  19. — The  Earth,  as  seen  from  the  Sun  at  the  Autumnal  Equinox, 
Sept.  22  (noon  at  London). 

summer,  and  through  autumn  to  winter  again  ;  the 
positions  of  the  earth  in  spring  and  autumn  when  the 
days  and  nights  are  equal,  are  called  the  equinoxes, 
that  is,  the  "  equal  nights." 

84.  You  will  also  be  able  to  see  that  during  the  sum- 
mer in  the  northern  hemisphere  the  sun  is  continually 


ASTRONOMY.  33 


visible  above  the  horizon  at  places  surrounding  the 
north  pole  ;  for  instead  of  setting  in  the  west,  it  goes 
apparently  round  by  north  to  east  again  above  the 
horizon  ;  and  in  winter  it  is  continually  below  the 
horizon,  never  rising  at  all.  In  the  southern  hemi- 
sphere the  same  thing  happens,  so  at  the  poles  there 
is  a  day  of  six  months  succeeded  by  a  night  of  the 
same  length. 

85.  I  have  given  four  drawings  of  the  earth  as  seen 
from  the  sun  in  Spring,  Summer,  Autumn,  and 
Winter.  The  centre  of  each  diagram  represents  the 
point  over  which  the  sun  is  at  the  different  times  of 
the  year.  Imagine  the  globe  to  turn  once  round  in 
each  of  these  positions,  and  what  I  have  told  you  will 
be  much  clearer. 


§  X.— THE  SEASONS  DEPEND  UPON  THE  DIF- 
FERENCE IN  THE  LENGTHS  OF  THE 
DAY  AND  NIGHT. 

86.  If  you  have  really  understood  why  the  day  and 
night  are  of  unequal  length  you  have  really  understood 
also  how  it  is  that,  both  in  England  and  Australia, 
there   is   winter  and   summer,    the    English  summer 
happening  at  the  same  time  as  the  Australian  winter ; 
why  in  fact  on  the  earth  the  seasons  change, 
and  we    have    the    succession    of    Spring,    Summer, 
Autumn,    and   Winter,    in    both    the    northern    and 
the   southern   hemisphere,  (that  is,  the  half   of    the 
earth  north  or  south  of  the  equator)  and  at  different 
times  of  the  year. 

87.  When  the  days  are  long  and  the  nights  are  short 
in  either  the  northern  or  the  southern  hemisphere,  in 


34 


SCIENCE  PRIMERS. 


[§xi. 


that  hemisphere  the  sun  is  visible  in  every  twenty- 
four  hours  for  a  longer  period  than  it  is  absent, 
therefore  the  heat  accumulates.  On  the  other  hand, 
when  the  days  are  short  and  the  nights  are  long  in 
either  hemisphere,  the  sun  is  absent  for  a  longer  time 
than  it  is  present,  so  the  absence  of  the  heat  is  more 
felt. 


FIG.  2a— Explanation  of  the  Seasons. 

88.  In  spring,  although  the  days  and  nights  are  equal 
as  in  autumn,  the  powers  of  nature  are  renewed  by 
their  winter's  rest,  so  spring  is  the  time  of  buds,  while 
autumn  is  the  time  of  decay. 


ASTRONOMY.  35 


§  XL— WHY  THE  MOVEMENTS  OF  THE  SUN 
AND  STARS  APPEAR  DIFFERENT  IN 
DIFFERENT  PARTS  OF  THE  EARTH. 

89.  I  must  now  endeavour  to  explain  how  it  is  that, 
as  seen  from  different  parts  of  the  earth,  the  motions 
of  the  heavenly  bodies  appear  to  be  very  different. 

90.  Not  only  at  the  poles  is  there  a  day  and  a 
night,  of  six  months,  and  not  only  at  the  equator. are 
the  days  and  nights  always  equal,  but  at  the  poles  the 
stars  seem  to  travel  round  a  point  overhead,  while  at 
the  equator  the  stars  which  travel  overhead  seem  to 
rise  and  set  almost  vertically,  and  not  on  a  slant  as 
they  do  in  England,  America,  and  Australia. 

91.  We    have    already  become    acquainted  with 
risings  and  settings  as  seen  here,  but  let  us  observe 
the  stars,  not  east  and  west,  but  in  other  parts  of 
the  sky,  and  see  how  they  move;  you  will  see  that  in 
England  the  stars  near  the  south  rise  only  a  little 
east  of  the  south,  get  to  the  highest  point  above  the 
horizon  exactly  south,  and  set  as  far  west  of  south  as 
they  rose  east  of  it.     Those  that  we  at  first  see  rising 
in  the  east,  pass  over  the  south  much  higher  above 
the  horizon,  and  set  in  the  west  again.     The  stars 
near  the  north  neither  rise  nor  set,  never  going  below 
the  horizon,  but  moving  in  circles  round  a  point  in 
the  heavens,  marked  by  a  star  called  the  pole  star, 
a  star  easily  found  by  its  being  pointed    at    by  the 
pointers  of  the  Great  Bear,  as  shown  in  the  diagram 

(Fig:  21). 

92.  Now,  to  illustrate  this,  take  a  small  globe,  make 
its  axis  upright,  and  in  order  to  indicate  the  horizon 

5 


SCIENCE  PRIMERS. 


[§xi. 


of  any  place  quite  plainly,  cut  a  piece  of  card  about 
the  size  of  a  penny  and  gum  the  centre  of  it  on  the 


FIG.  2i. — The  Pole  Star  and  the  Constellation  of  the  Great  Bear,  in  four 
different  positions,  after  intervals  of  six  hours,  showing  how  the  Great 
Bear  appears  to  travel  round  the  Pole  Star. 

globe  as  near  the  upper  axis  or  north  pole  as  the 
mounting  will  permit,  or  put  it  on  the  axis  if  you  can  ; 
then  a  person  standing  at  or  near  the  pole  would  be 
able  to  see  everything  above  the  card,  but  not  below — 
in  fact,  the  edge  of  the  card  represents  the  horizon. 
Now  spin  the  globe  to  represent  the  motion  of  the 
earth,  and  watch  what  the  appearance  of  the  stars  re- 
presented by  the  pictures  on  the  walls  (Art.  53)  would 
be  to  a  person  standing  at  the  pole.  You  will  at  once 
see  that  the  card  simply  turns  round  like  a  wheel,  and 
the  pictures  that  were  above  it  at  first  remain  so.  So 
the  stars  would  not  rise  or  set  to  a  person  at  the  pole, 


ASTRONOMY.  37 


but  remain  at  the  same  height  above  the  horizon,  and 
only  apparently  move  round  the  points  of  the  com- 
pass ;  the  pole  star  being  of  course  overhead,  and  the 
stars  turning  in  circles  round  it.  If  you  fix  on  a  picture 
on  the  walls  below  the  plane  (Art.  67)  of  the  piece 
of  paper  to  represent  the  sun,  you  will  see  you  cannot 
make  it  appear  to  rise  or  set  by  turning  the  globe 
round,  it  can  only  be  thrown  above  the  horizon  by 
tipping  down  the  globe  as  is  done  to  represent  the 
seasons.  Now  you  will  recollect  that  for  one  half  of 
the  year  the  north  pole  of  the  earth  is  tipped  towards 
the  sun,  and  during  the  other  half  away  from  the  sun, 
so  that  it  can  only  have  day  during  the  summer  half 
of  the  year,  and  night  during  all  the  winter;  and  if  you 
will  look  at  Fig.  20  you  will  see  that  during  the 
summer  the  whole  of  the  small  circle  round  the  pole 
is  lighted,  so  that  there  is  no  night  there  as  the  earth 
turns  round,  and  in  winter  for  the  same  reason  there 
is  no  day,  but  in  spring  and  autumn  half  the  circle  is 
light  and  half  dark,  so  that  every  place  is  brought  by 
the  turning  of  the  earth  into  daylight  and  back  into 
night  every  twenty-four  hours. 

93.  So  much  then  for  the  view  of  the  heavens  at 
the  pole.  Now  let  us  examine  what  takes  place  at  the 
equator.  To  do  this,  gurn  the  disc  of  card  on  the 
equator,  and  turn  the  globe.  You  will  see  that  it 
no  longer  turns  like  a  wheel,  but  turns  somewhat 
as  a  penny  does  when  spun  on  its  edge ;  and  on 
turning  the  globe  half-way  round,  an  entirely  new  set 
of  stars  appears  above  the  horizon,  represented  by 
the  edge  of  the  card,  the  two  places  in  the  heavens 
pointed  to  by  the  poles  6i  the  globe  will  be  just  on 
the  horizon,  the  north  pole-star  just  on  the  northern 


38  SCIENCE  PRIMERS.  [§  xi. 

part  of  the  horizon,  and  the  south  pole  just  on  the 
southern  part  of  the  horizon,  and  the  stars  which 
rise  due  east  will  pass  exactly  over  the  paper,  and  set 
due  west  as  the  globe  is  turned. 

94.  If  you  fix  on  one  picture  to  represent  the  sun, 
you  will  see  that  the  globe  can  be  just  turned  half- 
way round  while  the  sun,  or  the  picture  representing 
it,  is  above  the  paper  horizon,  and  half-way  round 
while  it  is  below  it ;  and  as  the  earth  turns  round  once 
every  twenty-four  hours,  the  sun  will  be  twelve  hours 
above  and  twelve  below  the  horizon,  so  the  day  and 
night  at  the  equator  are  always  of  equal  length,  and 
by  tipping  the  globe   to  represent  the   changes   of 
seasons  you  will  find  that  the  length  of  a  day  or 
night  remains  unaltered. 

95.  Now  try  for   yourself,  and  place  the  card  in 
other  positions  on  the  globe,  beginning  at  the  equator 
and  going  up  to  the  north  pole,  and  watch  the  gradual 
change   in  the  apparent  movements  of  the  stars  in 
rising  and  setting. 

96.  All  that  has  been  said  refers  to  the  apparent 
motions  of  the  stars  as  seen  on  the  equator,  or  to  the 
north  of  it;  so,  in  order  to  examine  the   apparent 
motions  of  the  stars  visible  in  the  southern  hemisphere, 
you  must  stick  the  card  at  different  places  south  of 
the  equator  of  the   globe,  and  turn   the  globe  and 
observe  what  takes  place.      First  place   it   between 
the  equator  and  south  pole,  to  represent  the   posi- 
tion of  an  observer  in  Australia,  then  the   equator 
will  be  north  of  him  instead  of  south,  and  his  pole 
south  instead  of  north,  as  in  our  hemisphere,  and  if  he 
looks  towards  the  north  he  will  see  exactly  the  same 
rising  and  setting  of  the  stars  as  he  would  in  the 


ASTRONOMY.  39 


northern  hemisphere  ;  but  his  right  hand  will  be 
towards  the  east  and  his  left  towards  the  west,  so  that 
the  stars  will  rise  on  his  right  hand  and  set  on  his 
left,  traversing  the  heavens  in  an  exactly  opposite 
direction  to  that  they  take  in  the  northern  hemi- 
sphere. Further,  he  will  see  near  the  northern  horizon 
the  stars  seen  in  England  near  the  southern  horizon, 
the  northern  stars  being  altogether  invisible  to  him. 

97.  In  order  to  make  the  apparent  movements  of 
the  stars  visible  in  the  southern  hemisphere  more  plain, 
call  the  upper  pole  of  the  globe  south,  and  the  lower 
north,  and  turn  the  globe  contrary  to  the  way  in  which 
you  turned  it  before ;  for  the  earth  appears  to  revolve 
in  a  different  direction  according  to  the  position  from 
which  it  is  viewed,  like  the  hands  of  a  watch,  for 
they  go   in  one  direction  if  looked  at  on  the  face, 
and  in    the    contrary  direction   if  looked  at  on  the 
back,  supposing  the  watch  to  be  transparent;  so  to 
an  observer   in   the   southern   hemisphere  the  earth 
appears  to  rotate  in  the  opposite  direction  to  that 
as   seen  from   the  northern  hemisphere,  and  conse- 
quently, if  we  make  the  south  pole  the  uppermost 
we  must  reverse  all  the  motions  including  its  motion 
round  the  sun. 

98.  When  you  have  done  this,  bring  the  true  south 
pole  of  the  globe  to  the  top,  and  then  experiment 
with  the  paper  horizon  as  before. 

99.  On  the  globe  you  will  probably  find  a  "wooden 
horizon,"  this  represents  the  horizon  of  the  centre  of 
the  earth,  as  we  have  supposed  the  circumference  of 
the  card  disc  to  represent  the  horizon  of  a  place. 


40  SCIENCE  PRIMERS. 


II.— THE  MOON  AND  ITS  MOTIONS. 

§  L— THE  MOON  TRAVELS  AMONG  THE 
STARS. 

100.  You  have  now  become  acquainted  with  the 
form  of  the  earth  and  with  its  motions,  first  its  spin 
or  rotation  round  its  own  axis  in  twenty-four  hours, 
and  secondly  its  movement  round  the  sun,  which  it 
accomplishes  in  a  year. 

1 01.  We  have  also  seen  how  these  two  real  move- 
ments of  the  earth  give  rise  to  two  apparent  motions 
of  the  sun  and  stars,  the  daily  movement  of  rising  and 
setting,  and  the  yearly  movement  by  virtue  of  which, 
month  after  month,  we  see  different  stars  in  the  south 
at  the  same  time  in  the  evening,  until,  after  the  expira- 
tion of  a  year,  the  grand  procession  begins  afresh. 
The  "Physical  Geography  Primer"  will  teach  you  what 
the  earth  is  like — that  it  is  a  cool  body  surrounded 
with  an  atmosphere  set  in  motion  by  the  sun's  heat. 

102.  Some  of  my  readers  will  wonder  why  as  yet  I 
have  said  nothing  of  the  moon,  which  appears  to  us 
almost   as   large  as   the  sun,  and  which   sometimes 
throws  such  a  strong  light  on  the  earth. 

103.  It  is  now  the  moon's  turn.     Look  at  it  some 
fine  evening,    and    notice    its   position  ariiongst  the 
neighbouring  stars;   it  is  difficult  to  see  small  stars 
near  it,  so  it  is  best  to  take  an  opportunity  when  it  is 
near  a  large  one.     Observe  it  again  some  hours  after- 
wards, or  if  need  be,  on  the  following  evening ;  you  will 
at  once  see  that  it  no  longer  occupies  the  same  position 
among  the  stars,  but  that  it  has  moved  among  them 


ASTRONOMY,  41 


considerably  towards  the  east.  It  will  be  observed 
to  rise  later  and  later  every  day,  by  three  quarters  of 
an  hour  to  an  hour,  as  is  easily  noticed  by  timing  its 
rising  for  a  few  successive  days.  It  keeps  on  losing, 
as  it  were,  on  the  sun,  till,  from  being  seen  at  sunset, 
it  does  not  rise  till  just  before  the  sun  in  the  morning. 
After  this,  the  sun  apparently  passes  it,  and  a  few 
evenings  afterwards  it  is  again  seen  in  the  west  just 
after  sunset,  only  to  lose  on  the  sun  and  be  over- 
taken again  every  twenty-eight  days  as  before,  in  the 
same  manner  as  the  hour-hand  of  a  clock  is  overtaken 
and  passed  by  the  minute-hand. 

104.  We  have  now  made  our  observations  :  let  us 
see  how  they  can  be  explained.     We  must  return  to 
our  orange  and  lamp,  and,  in  addition,  shall  require 
a  much  smaller  orange  to  represent  the  moon.     Now 
keep  the  orange,  representing  the  earth,  still,   and 
move  the  small  one  representing  the  moon  in  a  circle 
round  it.  as  the  earth  moves  round  the  sun. 

105.  We  have  to  see  if  this  motion  will  account 
for  our  observations.      First,  let  the  moon  be  at  E 
(Fig.   22),  in  a  line  with  the  sun,  and  as  in  such  a 
position  it  would  clearly  appear  to  us  to  be  in  the 
sky  near  the  sun,  then  it  will  appear  to  rise  and  set 
at  the  same  time  as  the  sun  does,  and  on  twisting 
the  earth  round  on   its  knitting-needle,  this  will  at 
once  be  clear.     Next  move  the  moon  to  T  to  re- 
present its  position  a  few  days  later;   you  will  now 
see  that  the  sun  will  set  some  time  before  the  moon, 
for  to  a  person   at  A   the  sun  is  just  set,  but  the 
moon  is  above  the  horizon.     Again,  move  the  moon 
to  F,  and  you  will  see  it  is  just  south  of  the  observer 
at  A,  when  the  sun  has  set,  so  that  it  has  lost  about 


42  SCIENCE  PRIMERS.  [§  n. 

six  hours  on  the  sun.  Move  it  further  on  to  G,  and 
it  will  just  be  rising  when  the  sun  is  setting,  and  will 
be  south  at  midnight,  having  lost  twelve  hours  on  the 
sun,  as  will  be  seen  supposing  an  observer  to  be  at  D ; 
move  the  moon  further  to  H,  then  to  the  observer  at  A, 
to  whom  the  sun  has  just  set,  the  moon  will  not  have 
risen ;  having  lost  eighteen  hours  on  the  sun,  it  will 
rise  at  mid-night,  as  will  be  seen  by  the  observer  at  D. 
To  the  observer  at  C,  the  moon  is  southing  and  the 
sun  is  rising ;  move  it  on  further  to  K^  it  will  nearly 
have  lost  a  whole  revolution  on  the  sun,  and  will 
rise  about  twenty-one  hours  after  it,  if  we  reckon  from 
the  time  they  both  rose  together  (or  three  hours  be- 
fore it,  if  we  reckon  the  other  way),  and  in  two  or 
three  more  days  they  will  both  rise  together  again. 
Now  it  is  clear  from  what  we  have  seen  that  its  losing 
on  the  sun  may  be  accounted  for  by  supposing  it  to 
travel  round  the  ,  earth  in  about  twenty-eight  days. 
And  this  we  know  to  be  the  case. 

§  II.— THE  MOON  CHANGES  HER  FORM. 

1 06.  We  have  thus  explained  the  moon's  own  motion 
among  the  stars,  but  something  else  happens  to  her : . 
as  she  moves  round  us,  she  changes  her  form  from  a 
crescent  to  a  circle.  These  changes  have  become  so 
familiar  to  us,  having  heard  of  the  changes  of  the 
moon  as  far  back  as  we  can  remember,  that  we  are 
apt  to  look  on  them  as  a  matter  of  course,  without 
inquiring  into  their  cause.  Let  us  ask  the  question, 
"  Does  the  moon  really  change  ?  "  No,  it  is  always 
there,  but  a  portion  is  sometimes  unillumi- 
nated  and  invisible  to  us. 


ASTRONOMY.  43 


107.  Observe  the  moon  some  evening ;  suppose  you 
see  it  at  the  "full  moon"  as  it  is  called,  when  it  appears 
round,  like  the  sun  :  observe  whereabouts  it  is  in  the 
sky,  and  you  will  find  that  it  is  on  the  opposite  side  of 
the  earth  to  the  sun,  and  that  it  consequently  rises  at 
sunset  and  sets  at  sunrise,  in  fact  it  is  in  position  G 
(Fig.  22);  now  place  the  ball  representing -the  moon 
at  G  on  the  opposite  side  of  the  orange  to  the  sun, 
then  the  half  of  tire  ball,  which  is  white  in  the  diagram, 


a 

N 


LAMP   OR    SUN 


?    9 


?  o 

r  <i  o 

a 

FIG.  22.— The  Moon's  motion  round  the  Earth. 

will  be  illuminated  by  the  sun,  and  the  other  half,  op- 
posite to  it,  will,  of  course,  be  dark,  in  the  same  manner 
as  we  have  night  when  the  sun  is  shining  on  the  other 
side  ot  the  earth  to  us,  and  if  you  place  your  eye 
near  the  orange,  you  will  see  all  the  bright  portion 
and  none  of  the  dark  side ;  it  is  then  full  moon,  and 
this  appearance  is  represented  by  the  white  circle  M. 
So  that  it  is  now  clear  that  at  full  moon  the  moon  is 


44  SCIENCE  PRIMERS.  [§  11. 

on  the  opposite  side  of  the  earth  to  the  sun,  and  we 
see  therefore  the  bright  side. 

108.  After  the  full,  the  moon  rises,  as  we  have  seen 
before,  later  and  later  after  sunset,  and  we  will  suppose 
you  observe  it  a  week  after  the  "  full."    It  will  rise,  as 
you  will  find,  about  midnight.     Rather  late,  you  say, 
to  sit  up,  but  the  day  of  astronomers  is  other  people's 
night.     The  moon  now  is  no  longer  apparently  round, 
only  half  of  it  is  visible.     Return  to  the  diagram :  in 
what  position  is  the  moon  if  it  rises  at  midnight  ?    It 
is  midnight  to  an  observer  at  Z>,  and  the  moon  to  be 
rising   must   be  at  H>     Place  the  ball,  therefore,  at 
H,  and  the  eye  at  D ;  now   the  part,  white  in  the 
diagram,  is  the  bright  half  illuminated  by  the  sun ;  but 
in  this  position  the  whole  of  it  is  not  visible,  but  only 
half  of  it  and  half  of  the  dark  portion,  you  will  there 
fore  see  that  we  ought  to  have  the  appearance  of  half 
moon,  N,  in  this  case,  which  we  do  in  reality. 

109.  Let  us  continue  our  observations.     If  it  is  too 
late  to  sit  up  after  midnight,  try  and  get  up  before 
sunrise  and  you  will  see  that,  as  the  moon  is  appa- 
rently overtaken  by  the  sun,  it  will  get  more  and  more 
crescent  shaped,  and  when  at  K  it  appears  as  at  O,  till 
it  is  lost  in  the  sun's  rays  and  comes  to  position  E. 
How  ought  it  to  appear  now  ?    Place  the  ball  between 
the  eye  and  the  lamp,  and  you  will  see  the  whole  of 
the  dark  half  and  none  of  the  bright  portion.     It  is 
"  new  moon ; "  look  at  it  a  few  days  after,  when  it  will 
be  visible  just  after  sunset.     It  will  appear  in  a  thin 
crescent,  and  will  be  in  the  position  marked  T  in  the 
diagram.  Place  the  ball  in  this  position,  and  by  placing 
your  eye  close  to  the  orange  you  will  see  just  a  crescent 
of  the  bright  half,  and  a  large  portion  of  the  dark  half. 


ASTRONOMY.  45 


1 10.  As  the  moon  appears  to  get  further  and  further 
from  the  sun,  and  to  set  later  and  later,  more  and 
more  of  the  bright  half  will  be  seen,  till  we  get  to 
half  moon  in  position  F.  It  is  now  south  at  sunset. 
Place  the  ball  in  this  position,  and  your  eye  close  to 
the  orange,  and  you  will  see  the  observation  is  ac- 
counted for.  Another  week  more  and  the  moon  again 
becomes  full,  and  opposite  the  sun. 

in.  All  these  observations  may  be  thoroughly 
mastered  by  standing  at  a  distance  from  the  lamp,  or 
gas-light,  which  should  be  the  only  one  in  the  room, 
and  moving  an  orange,  or  ball,  round  your  head, 
when  all  the  changes  of  the  moon  will  be  rendered 
clear  to  you.  The  moon,  therefore,  revolves 
round  the  earth  in  the  same  manner  as  the 
earth  goes  round  the  sun,  passing  from  full 
moon  to  full  moon  in  about  twenty-nine  and 
a  half  days. 

§  III.— HOW  THE  MOON  CAUSES  ECLIPSES. 

112.  From  what  we  have  seen,  you  might  think  that 
the  moon  ought  to  pass  between  us  and  the  sun  every 
month,  and  produce  what  is  called  a  total  eclipse 
of  the    sun  ;    but,   for  reasons  of  which  we  shall 
presently  speak,  it  sometimes  passes  a  little  above 
the  sun,  and  at  others  a  little  below,  when  there  is 
no  eclipse  at  all,  or  it  passes  over  a  part  only  of  the 
sun,  and  so  only  covers  a  portion  of  the  sun's  disc 
from  our  view,  producing  what  is  called  a  partial 
eclipse. 

113.  Let  us  see  if  we  can  make  matters  clear  with 
the  use  of  our  orange  and  ball. 


46  SCIENCE  PRIMERS.  [§  in. 

114.  Set  the  lamp  on  the  table,  and  stick  the  knitting 
needle  supporting  the  orange  into  a  large  pin  cushion 
at  some  distance  from  it;  then  take  the  small  ball 
representing  the  moon  and  suspend  it  by  a  string^  so 
that  you  can  move  it  round  the  earth  (Fig.  23),  without 
the  fingers  casting  a  shadow  on  it.  Now  bring  the 
moon  between  the  sun  and  earth,  holding  it  near  the 
earth  as  at  C  (Fig.  23),  so  that  the  shadow  of  the  moon 
falls  on  the  earth :  wherever  this  shadow  falls  on  the 
earth  there  will  the  sun  be  invisible,  and  there  will  be 
a  total  eclipse  at  that  place.  At  other  places  on 
the  earth,  as  at  J3,  which  the  darkest  part  of  the  shadow 


FIG.  23.— Total  Eclipse  of  the  Sun. 

does  not  reach-,  the  whole  of  the  sun  will  not  be  covered 
by1  the  moon.  Here,  then,  we  shall  have  only  a  partial 
eclipse,  and  the  further  you  go  from  this  region  the 
more  of  the  sun  will  be  visible,  so  that  round  the 
total  shadow  is  another  kind  of  half  shade,  called  the 
penumbra,  and,  as  we  have  seen,  all  places  inside 
the  penumbra  will  see  a  partial  eclipse  only. 


ASTRONOMY. 


47 


115.  Now  move  the  moon  further  away  from  the 
earth,  to  say  D  (Fig.  24),  and  you  will  see  that  the 
shadow  of  the  moon  is  not  sufficiently  long  to  reach 
the  earth,  so  there  can  be  no  total  eclipse,  the  moon 
being  so  far  away  that  its  disc  is  not  sufficiently  large 
to    cover  the  sun  completely,  so  there   remains  the 
outside  edge  of  the  sun  visible  ;  this  sort  of  eclipse 
is  called  an  annular  eclipse. 

1 1 6.  All  this  will  be  clearer  if  the  orange  be  re- 
moved and  the  eye  placed  in  its  stead.     First  place 


FIG.  24.— Annular  Eclipse  of  the  Sun. 

your  eye  where  the  shadow  was  (Fig.  24),  that  is,  in  the 
umbra  of  the  moon,  and  you  will  see  a  total  eclipse. 
Then  move  the  eye  a  little  lower,  still  keeping  the 
moon  in  the  same  place,  and  you  will  see  a  crescent 
of  the  sun,  in  fact  a  partial  eclipse,  and  the  further  you 
move  your  eye  from  it,  the  more  of  the  sun  you 
will  be  able  to  see.  Now  place  the  eye  at  A  and  so 
see  a  total  eclipse,  and  move  the  moon  gradually  away 
from  you,  and  you  will  see  the  moon  apparently 


48  SCIENCE  PRIMERS.  [§  in. 

getting  smaller,  so  that  at  D  (Fig.  24),  it  is  no  longer 
large  enough  to  cover  the  sun,  and  you  see  the 
bright  edge  of  the  sun  round  the  moon ;  in  fact,  an 
annular  eclipse. 

117.  Besides  eclipses  of  the  sun,  there  are  eclipses 
of  the  moon,  occasioned  by  the  moon  passing 
through  the  shadow  of  the  earth.  You  will  readily 
understand  how  these  happen  by  placing  the  lamp 
and  orange  as  before  :  on  passing  the  ball,  repre- 
senting the  moon,  round  on  the  opposite  side  of  the 
earth  to  the  sun,  it  will  go  into  and  through  the 
shadow  of  the  earth,  and  will  be  darkened,  not,  as 


FIG.  25.— Eclipse  of  the  Moon. 

in  the  case  of  an  eclipse  of  the  sun,  by  an  opaque 
body  coming  between  us  and  the  sun,  but  by  its 
being  shaded  by  our  earth  (Fig.  25). 

1 1 8.  To  an  observer  on  the  moon  during  a  total 
•eclipse  of  the  sun,  the  earth  would  appear  to  have 
a  black  spot  on  it,  moving  rapidly  across  it;   and 
surrounding  the  spot  would  be  a  circle  of  half  shade, 
the  penumbra,  in  which  a  partial  eclipse  is  seen  from 
the  earth ;  but  in  the  case  of  a  total  eclipse  of  the 
moon,  the  shadow  of  the  earth  entirely  envelopes  the 
moon. 

119.  You  will  have  understood  by  this  time  that  an 


ASTRONOMY.  49 


eclipse  of  the  sun  can  only  take  place  at  new 
moon,  and  an  eclipse  of  the  moon  can  only 
take  place  at  full  moon.  The  reason  being  that 
when  the  moon  is  between  us  and  the  sun,  that  is, 
when  an  eclipse  of  the  sun  can  happen,  the  moon's  dark 
side  will  necessarily  be  turned  towards  us ;  and  when 
the  moon  is  on  the  other  side — on  the  opposite  side 
of  us  to  the  sun,  that  is,  when  an  eclipse  of  the  moon 
can  happen,  it  must  have  its  bright  side  towards  us. 

1 20.  We  have   spoken   (Art.    112)   of  the   moon 
passing  sometimes  above,  and  at  other  times  below 
the  line  joining  the  earth  and  sun,  and,  as  you  will 
see  by  referring  to  the  orange  and  ball,  an  eclipse  of 
the  sun  and  another  of  the  moon  must  happen  every 
month  if  the  moon  did  not  so  pass. 

121.  Let  us  see  how  we  can  account  for  the  fact 
that  the  moon  does  thus  pass  sometimes  above  and 
sometimes  below  the   sun,  thus  preventing  monthly 
eclipses.    We  have  found  that  the  moon  revolves  round 
the  earth  in  nearly  a  circle  (with  the  earth  at  the  centre) 
called  its  orbit  or  path.     Let  us  represent  this  orbit 
by  a  piece  of  wire,  bent  in  a  circle  round  the  orange, 
and  let  the  moon  be  represented  by  a  large  bead  or  a 
small  ball  strung  on  it.     Hold  the  ring  of  wire  so  that 
the  earth  (orange)  is  in  the  centre,  and  move  the  moon 
on  the  wire  round  it,  and  you  will  find  that  if  the  ring 
is  held  horizontally  the  moon  will  pass  between  the 
earth  and  sun,  represented  as  usual  by  the  lamp,  at 
every  revolution.     Now  this  we  have  observed  is  not 
the  case  with  the  real  moon,  and  in  order  to  make  the 
bead  pass  above  or  below,  the  part  of  the  ring  between 
the  lamp  and  the  orange  must  be  tipped  up  or  down. 

122.  To  make  this  clearer,  get  a  tub  of  water  as 


SCIENCE  PRIMERS. 


before,  and  float  in  the  middle  a  ball  to  represent  the 
sun,  so  that  half  is  above  water  and  half  below.  Float 
another  small  ball  near  the  side  of  the  tub  to  repre- 
sent the  earth,  then  the  earth  can  be  floated  round 
the  sun,  to  represent  its  annual  path.  Now,  as  its 
orbit  will  lie  on  the  surface  of  the  water,  this  surface, 
as  we  have  seen  before  (Art.  67),  represents  t-he 
plane  of  the  ecliptic. 

123.  But  we  have  already  suspected  that  the 
moon's  orbit  is  inclined  to  this  plane,  so  that  at 
certain  times  no  eclipse  takes  place ;  and  if  we  take  the 


FIG.  26.— Shewing  the  inclination  of  the  Moon's  orbit  to  the  plane  of  the 
ecliptic. 

wire  ring  as  before,  to  represent  the  moon's  orbit,  and 
place  it  round  the  earth,  dipping  one  half  of  the  ring 
below  the  surface  of  the  water,  and  keeping  the  other 
above,  as  represented  in  Fig.  26,  where  the  full  line 


ASTRONOMY.  51 


indicates  the  part  above  water  and  the  dotted  line  the 
part  below,  we  represent  the  inclination  of  the  moon's 
orbit  to  the  plane  of  the  ecliptic,  and  the  line  joining 
the  points  where  the  orbit  cuts -this  plane  is  called 
the  line  of  nodes,  and  B  and  D  are  the  nodes. 

124.  This  will  render  it  clear  that  eclipses,  suppos- 
ing the  orbit  of  the  moon  to  be  inclined  to  the  plane 
of  the  ecliptic,  could  only  happen  when  the  moon  is 
at  the  part  of  its  orbit  near  a  node  when  she  comes 
in  a  line  with  the  earth  and  sun,  for  only  then  does 
she  in  her  revolution  pass  between  the  sun  and  the 
earth.     At  the  other  parts  of  the  orbit  there  can  be 
no  eclipse,  because  the  bead  on  the  ring  would  at  its 
nearest  approach  to  an  eclipse  be  below  or  above  the 
water,  and  not  on  its  surface  in  a  line  with  sun  and 
earth.     And  as  eclipses  do  not  happen  every  month 
we  know  that  the  moon's  orbit  is  inclined  as  we  have 
supposed  it  to  be. 

125.  We  have  seen  before  that  the  plane  of  the 
earth's  motion  round  its  axis  is  inclined  to  the  plane 
of  the  ecliptic,  and  we  now  find  that  the  plane  of  the 
moon's  motion  round  the  earth  is  inclined  to  the  same 
plane.     We  should  now  endeavour  to  understand  ho\V 
the  amount  of  inclination  is  fixed  in  each  case. 

126.  To  do  this  astronomers  divide  all  circles,  whe- 
ther large  or  small,  into  360  degrees  (written  360°), 
(see  Fig.  27),  and  if  we  draw  two   lines   from   the 
centre  of  a  circle  to  the  circumference  the  number  of 
degrees  intercepted  between  the  points  where  they  cut 
the  circumference  is  the  measure  of  the  angle  between 
the  two  lines  at  the  centre.     Now  360  is  four  times 
QO,  so  that  two  lines  containing  a  quarter  of  a  circle 
make  an  angle  ot  90°  between  them.     You  will  see 


52  SCIENCE  PRIMERS.  [§  in. 

that  the  size  of  the  circle  is  of  no  consequence,  for  if 
you  draw  a  number  of  circles,  one  inside  the  other, 
all  having  the  same  point  for  their  centre,  and  from 
the  centre  draw  two  lines  intercepting  a  quarter  or 
90°  of  the  outer  circle,  then  you  will  see  that  it  inter- 
cepts also  a  quarter  of  each  of  the  others.  Each  90° 
is  called  a  right  angle,  and  two  lines  which  make  an 


FIG.  27. — Division  of  the  circle  into  degrees. 

angle  or  opening  of  90°  between  them  are  said  to  be 
perpendicular  to  each  other.  A  complete  circle  like 
this  is  contains  360  angles  of  i°,  4  angles  of  90°,  and 
so  on. 

127.  Now  astronomers  conceive  such  a  circle  with 
its  centre  at  the  centre  of  the  earth,  and  they  can 
than  by  their  observations  determine  the  angles 


ASTRONOMY.  53 


formed  by  the  planes  to  which  we  have  referred  in 
Art  125;  and  they  have  thus  found  that  the  angle 
made  by  the  plane  of  the  ecliptic,  and  the  plane  of 
the  earth's  motion  of  rotation  is  23°,  or  thereabouts ; 
and  the  angle  made  by  the  plane  of  the  ecliptic  and 
the  plane  of  the  moon's  motion  round  the  earth,  is 
a  little  over  5°. 

§  IV.- WHAT   THE   MOON    IS   LIKE. 

128.  I  have  already  referred  to  the  teachings  of 
Physical  Geography  with  regard  to  the  Earth.     The 
moon  is  near  enough  to  us,  being  only  some  quarter 
of  a  million  of  miles  away,  to  enable  us  to  learn  much 
about  its  surface. 

129.  If  the  moon  be  looked  at  with  the  unaided 
eye  its  surface  appears  mottled,  some  portions  being 
darker  than  others;    and  those    darker  places  were 
thought  by  the  ancients  to  be  seas,  and,  although  they 
have  since  been  found  to  be  dry  land,  they  still  retain 
the  name  of  seas  :  so   we   have    "  Sea  of  serenity," 
"Sea  of  storms,"  and  the  like,   as  you  will  see  on 
looking  at  a  map  of  the  Moon,  for  we  have  a  map  of 
the  Moon  as  we  have  a  map  of  the  Earth.     If  you 
employ  a  telescope  to  aid  the  eye — and  a  small  one 
will  answer  the  purpose, — the  surface  is  seen  to  be 
almost  completely  covered  with  mountains,  hills,  and 
valleys,  but  not  altogether  mountains  and  valleys  as 
we   have   them  here,  covered  with  verdure,  but  all 
dry  and  barren.     There  are  no  lakes  or  rivers,  and, 
as  far  as  is  yet  known,   there  is  no  water  whatever, 
and  consequently  no  clouds  to  shade  the  surface  from 
the  sun;  and  what  is  more,  there  is  no  appreciable 


54  SCIENCE  PRIMERS.  [§  iv. 

atmosphere.  Hence  there  is  probably  no  life  on  the 
moon.  Nearly  the  whole  surface  is  covered  with  ex- 
tinct volcanoes  of  enormous  extent,  and,  unlike  those 
you  read  of  on  the  earth. 

130.  You  will  see  from  these  facts  about  the  moon 
how  the  conditions  of  the  planet  on  which  we  dwell 
may  not  apply  to  the  other  bodies  in  the  skies.    Fancy 
a  world  without    water,   and   therefore    without  ice, 
cloud,  rain,  and  snow,  without  rivers  and  streams, 
therefore  without  vegetation  to  support  animal  life  : 
a  world  without  twilight  or  any  gradations  between 
the  fiercest  sunshine  and  the  blackest  night ;  a  world 
also  without  sound,  for  as  sound  is  carried  by  the  air 
the  highest  mountain  on  the  airless  moon  might  be 
riven  by  an  earthquake  inaudibly  ! 

131.  You  will  recognize,  too,  that  the  moon  must 
resemble  the  earth  in  this :  it  does  not  shine  by 
its  own  light.     The  bright  part  of  the  moon  is  that 
on  which  the  sunlight  falls ;  where  this  light  does  not 
fall  the  moon  is  invisible :  hence  moonlight  is  sunlight 
second-hand,  and  the  moon  does  not  give  us  light  of 
its  own. 

132.  The  diameter  (Art.  22)  of  the  moon  is  about 
2,000  miles ;  and,   bulk  for  bulk,  its   materials  are 
lighter  than  those  of  which  the  earth  is  built  up. 
This  is  expressed  by  saying  that  the  density  of  the 
moon  is  f ,  that  of  the  earth  being  i. 

133.  Now  this  requires  a  little  explanation.     You 
know  that  some  things  are  very  dense  and   heavy, 
others  are  very  light ;  lead  for  instance  is  very  dense 
and    heavy,  cork  is   very   light.      Now   you   know 
what  an   inch  is,  and   a  square  inch,  and  a  cubic 
inch.     Suppose  that  you  took  a  cubic  inch  of  lead, 


ASTRONOMY.  55 


and  a  cubic  inch  of  cork,  then,  by  weighing  them 
both,  you  would  be  able  to  tell  exactly  how  much  the 
lead  was  heavier  than  the  cork.  Calling  the  weight 
or  density  of  the  cork  i,  the  weight  or  density  of  the 
lead  would  be  so  and  so.  And  of  course  if  you  took 
instead  of  a  cubic  inch,  a  cubic  yard  or  a  cubic  mile, 
the  lead  would  weigh,  exactly  the  same  number  of 
times  more  than  the  cork. 

134.  Astronomers  have  found  out  the  weight  of  the 
earth,  and  of  the  moon,  and  they  also  know  how 
many  cubic  miles  (or  cubic  inches)   each   contains. 
They  can  therefore  easily  find  whether  a  cubic  inch  or 
mile  of  the  materials  of  which  the  moon  is  built  up 
weighs  less  or  more  than  a  cubic  inch  or  mile  of  the 
materials  of  which  the  earth  is  built  up ;   in  other 
words,  whether  the  earth  is  less  or  more  dense  than 
the  moon.     And  they  have  found  that  a  cubic  inch  of 
the   earth's  materials  weighs    ij  times   as   much   as 
a  similar  quantity  of  the  moon's  materials,  hence  they 
say  that  the  moon  is  only  J  as  dense  as  the  earth. 

135.  More  .commonly   the   weight   or   density   of 
a  cubic  inch  of  water  is  taken  as  i,  then  we  say  that 
the  density  of  the  earth  is  5^,  and  that  of  the  moon 
3 1  times  greater  than  that  of  water.     Thus  then  we 
have  in  the  case  of  each  celestial  body  : 

a.  Its  volume  expressed  in  cubic  miles  or  cubic 
inches  determined  from  its  diameter. 

b.  Its  weight  or  mass,  that  is  to  say  how  many 
tons  it  weighs,  this  is  determined  from  its  action  on 
other  bodies. 

c.  Its  density,  that  is  how  much  a  cubic  inch  or 
cubic  mile  weighs  \  this  is  found  by  dividing  its  mass 
or  weight  by  its  volume. 


56  SCIENCE  PRIMERS.  [§  i. 

136.  The  same  side  of  the  moon  is  always  turned 
towards  us,  for  as  the  moon  goes  round  the  earth  it 
slowly  turns  on  its  own  axis,  and  makes  one  revolution 
in  exactly  the  same  time  as  it  takes  it  to  get  round 
us,  just  in  the  same  way  as  you  would  do  if  you  were 
to  take  hold  of  a  pole  stuck  in  the  ground,  with  your 
hands,   and  go  round   it,  always  keeping  your  face 
turned  towards  the  pole.     You  would  then  see,  by 
looking  at  adjacent   objects,  that  you  turned  round 
once  every  time  you  went  round  the  pole,  and  you 
will  probably  become  giddy,  thereby  giving  conclusive 
evidence  of  your  rotation. 

137.  It  follows  from  this  fact  that  the  moon  only 
turns    round   once  on  its  own  axis  during  each  re- 
volution round  the  earth,  and  that  the  lunar  days  are 
about  29  of  our  days.     We  are  lighted  by  the  sun  for 
about  12  hours,  or  the  half  of  24  hours ;  each  portion 
of  the  moon  is  lighted  for  about  14  days,  or  the  half  of 
29  days,  so  you  can  imagine  how  intensely  heated  the 
surface  must  become  during  the  lunar  day,  and  how 
cold  the  opposite  side  must  get  during  the  14  days' 
night. 


III.— THE  SOLAR  SYSTEM. 

I.— HOW  BODIES  LIKE  THE  EARTH,  NEARER 
THE  SUN,  WOULD  APPEAR  TO  US. 

138.  So  far  as  we  have  gone  the  earth  on  which  we 
dwell,  the  large  sun  and  moon,  and  the  tiny  stars,  are 
the  only  bodies  with  which  we  have  dealt. 


ASTRONOMY.  57 


139-  Let  us  see  what  we  should  observe  in  the 
heavens  if  there  were  other  bodies,  not  shining  by 
their  own  light — other  earths  like  ours,  revolving  round 
the  sun  as  we  do.  How  would  they  appear  to  us  ? 
And  first  let  us  take  the  case  of  a  body  travelling 
round  the  sun  but  at  a  less  distance  from  him  than  we 
are.  Let  us  think.  Take  the  lamp  to  represent  the 
sun,  the  orange  for  the  earth,  and  the  ball  used  for  the 
moon  to  represent  the  other  earth ;  then  all  we  have 
•to  do  in  order  to  represent  the  appearance  of  the  new 
world  in  its  journey  round  the  sun,  is  to  move  the  ball 
round  the  lamp,  and  see  how  it  appears  from  the 
orange  in  its  different  positions.  First  place  it  in  the 


4 


A 


FIG.  28.  —  Diagram  illustrating  the  motions  and  appearances  of  a  tody 
between  us  and  the  sun. 

position  represented  by  A,  Fig.  28,  between  the  lamp 
and  the  orange — then  it  will  appear  in  the  same  line 
with  the  sun,  and  accompany  the  sun  in  its  path 
across  the  sky,  at  which  time  of  course  it  will  be  in- 
visible on  account  of  the  superior  brightness  of  the  sun, 
but  it  will  set  and  rise  with  it ;  now  move  it  to  B — 
it  will  then  appear  on  the  right  side  of  the  sun,  and 
will  rise  before  daylight  and  set  before  the  sun,  so  that  it 
would  only  be  seen  before  sunrise,  changing  its  place, 
— "  wandering  "  among  the  stars  from  day  to  day  (the 
word  planet  means  a  "wanderer"),  to  be  put  out  like  the 
stars  by  the  day.  Move  it  to  position  C—  it  will  then  rise 


58  SCIENCE  PRIMERS.  [§  n. 

and  set  with  the  sun,  and  will  be  lost  in  the  sun's  rays 
as  at  A.  Again  move  it  to  D— it  is  then  on  the  left 
side  of  the  sun  and  will  rise  after  daylight,  and  set  after 
sunset,  so  that  it  will  be  seen  only  in  the  evening.  A 
little  consideration  will  make  it  plain  that  this  body  will 
go  through  the  same  changes  as  the  moon,  and  again 
that  we  can  never  see  it  at  midnight.  But  there  will  be 
an  important  difference.  As  we  go  round  the  sun,  keep- 
ing always  about  the  same  distance  from  the  sun,  the 
sun  always  seems  to  be  about  the  same  size ;  and  as 
the  moon  goes  round  the  earth,  keeping  about  the 
same  distance  from  it,  the  moon  always  seems  to  be 
about  the  same  size.  Mind,  I  do  not  say  the  same  form. 
But  the  new  earth  about  which  we  are  now  think- 
ing goes  round  the  sun;  so  it  will  sometimes  be 
between  us  and  the  sun  and  sometimes  on  the  opposite 
side  of  the  sun,  so  that  its  distance  from  us  will  vary ; 
therefore,  its  apparent  size  will  vary. 

140.  Hence,  if  we  were  to  examine  this  new  earth 
with  a  telescope,  we  should  see  it  vary  in  size  and  also 
in  shape  like  the  moon,  and  if  its  atmosphere  were 
clear,  we  should  see  its  seas  and  continents,  and  so 
by  their  motion  we  should  be  able  to  ascertain  how 
fast  it  turned  round  on  its  axis — whether  its  day  was 
longer  or  shorter  than  ours. 


§  II.— HOW  BODIES  LIKE  THE  EARTH,  FUR- 
THER OFF  FROM  THE  SUN,  WOULD 
APPEAR  TO  US. 

141.  In  order  to  represent  the  appearance  of  an 
earth  outside  us,  we  have  only  to  move  the  ball  in 
a  circle  round  the  sun,  outside  the  earth's  orbit.  Let 


ASTRONOMY. 


59 


us  begin  by  holding  the  ball  on  the  opposite  side  of 
the  sun  to  the  earth — then  it  will  be  lost  in  the 
sun's  rays,  and  on  moving  it  further  round  in  the 
contrary  direction  to  the  hands  of  a  clock,  it  will  be 
seen  on  the  left  side  of  the  sun,  and  will  therefore 
set  after  it  just  as  the  interior  earth  did ;  but  as  you 
move  it  on  after  it  has  made  a  quarter  of  a  revolution,, 
it  appears  to  recede  further  and  further  from  the  sun,, 


FIG.  29. — Diagram  illustrating  the  motion  of  a  body  travelling  round  the  suni 
outside  the  orbit  of  the  earth. 


instead  of  again  approaching  it,  and  passing  between 
the  earth  and  sun ;  and  eventually  it  comes  to  the 
opposite  side  of  the  earth  to  the  sun  and  rises  at 
sunset,  and  is  visible  in  the  south  at  midnight,  which 
as  we  have  seen  was  impossible  in  the  case  of  a 
body  between  the  sun  and  the  earth. 

142.  You  will  also  notice  that  nearly  all  the  bright 
side  is  visible   to   the    earth,    although  at   the   two. 
positions  corresponding  to  A  and  B,  Fig.  29,  it  will; 
7 


60  SCIENCE  PRIMERS.  [§  in. 

show  a  portion  of  its  dark  side,  so  that  an  exterior 
earth  would  not  go  through  all  the  changes  that  an 
interior  one  would  do.  While,  therefore,  the  interior 
earth  would  appear  to  swing  from  side  to  side  of 
the  sun,  only  the  exterior  one  would  take  a  sweep 
round  outside  our  earth.  Such  a  body  will  vary  its 
size,  but  not  to  so  great  an  extent  as  an  interior  one. 


§  III.— ARE  THERE  SUCH  BODIES?— THE 
PLANETS. 

143.  There  are  such  bodies  as  we  have  just  been 
considering,    both   interior    ones    and    exterior  ones, 
and  they  are  all  called    Planets,    and  the  earth  is 
called  a  planet  simply  because  it,  like  them,  would 
appear  to  wander  among  the  stars  to  astronomers  on 
the  other  planets,  if  such  there  be.      The  principal 
planets   are   eight   in   number,    including   our   earth. 
They  have  been  named  after  the  ancient  deities  ;  the 
two  interior  ones,  Mercury  and  Venus,  and  the  exterior 
ones,  Mars,  Jupiter,  Saturn,  Uranus,  and  Neptune  j 
the  three  first  being  smaller  than  our  earth,  and  the 
remainder  a  great  deal  larger. 

144.  Mercury  and  Venus  are  known  to  be  interior 
planets,    that  is,  planets   between   us   and   the    sun, 
because  they  appear  to  swing,  as  we  have  found  such 
bodies  should  do,  on  either  side  of  the  sun.     Mercury 
very  seldom  leaves  the  sun  sufficiently  to  rise  so  early 
before   the   sun,  or  set  so  late  after  him,  as  to  be 
visible.     Venus,  however,  gets  so  far  away  as  to  be 
seen  long  after  sunset  or  before  sunrise,  and  is  called 
the  Evening  or  Morning  star,  accordingly. 

145.  The  exterior  planets,  as  we  found  such  bodies 


ASTRONOMY.  61 


should  do,  make  a  complete  tour  of  the  heavens.  All 
these  movements  are,  however,  rather  more  com- 
plicated than  we  have  found  with  the  orange  and  ball, 
for  the  earth  is  not  fixed,  but  going  round  the  sun 
quicker  than  the  exterior,  and  slower  than  the  interior 
planets;  and,  in  order  to  represent  the  true  apparent 
motions  you  must  move  the  orange  round  the  sun  at 
a  rate  depending  upon  which  planet  you  wish  to  re- 
present by  the  ball. 

146.  The   sun   and   planets   revolving   round   him 
form  what   is   called  the   solar   system ;    in  fact, 
everything   over  which   the   sun   has   continued   in- 
fluence is  a  member  of  this  system. 

147.  Thus    besides    the   planets    there    are    other 
members  of  the  system,  namely,  comets  and  falling 
stars,  which  will  be  mentioned  again  more  fully  here- 
after :  all  these  bodies  form  a  sort  of  family  having 
the  sun  for  their  head,  and  on  Plate  II.  will  be  seen 
a   view   of  this    system    as   it    would   appear    when 
looked  at  from  above ;  but  it  is  impossible  thus  to 
give  an  idea  of  the  true  scale  of  the  system.     In  order 
to  do  this,  take  a  globe  a  little  over  two  feet  in  dia- 
meter to  represent  the  sun  :  Mercury  would  now  be 
proportionately  represented  by  a  grain  of  mustard-seed, 
revolving  in  a  circle  164  feet  in  diameter;   Venus 
a  pea,    in   a  circle   of    284  feet  in   diameter ;    the 
earth  also  a  pea,  at  a  distance  of  430  feet;  Mars,  a 
rather  large  pin's  head,  in  a  circle  of  654  feet ;  the 
smaller  planets  by  grains  of  sand,  in  orbits  of  from 
1,000  to  1,200  feet;  Jupiter,  a  moderate  sized  orange, 
in  a  circle  nearly  half  a  mile  across  ;  Saturn,  a  small 
orange,  in  a  circle  of  four-fifths  of  a  mile ;  Uranus, 
a  full-sized  cherry,  or  small  plum,  upon  the  circum- 


62  SCIENCE  PRIMERS.  [§  iv. 

————————— _ • . 

ference  of  a  circle  more  than  a  mile  and  a  half;  and 
Neptune,  a  good-sized  plum,  in  a  circle  about  two 
miles  and  a  half  in  diameter. 

148.  I   have    already   told   you   that    the    earth's 
distance  from   the  sun,  represented  in  Art.   147  by 
430  feet,  is  really  91  millions  of  miles.     I  cannot  give 
you  any  idea  of  this  distance.     I  can  only  state  tlm 
if  a  train  going  at  the  rate  of  thirty  miles  an  hour  were 
to  leave  the  earth  on  the  first  of  January,  1875,  it 
would  only  reach  the  sun  in  the  middle  of  the  year 
2213. 

149.  Beginning  with  this  rough  idea  we  will  now 
consider,  the  interior  planets— those,  namely,  which 
are  nearer  the  sun  than  the  earth. 


§  IV.— THE   INTERIOR  PLANETS. 
MERCURY. 

150.  Mercury,  the  nearest  planet  to  the  sun,  revolves 
round  him  at  a  distance  of  about  35  millions  of  miles  ; 
the  earth's  distance  from  the  sun  being  91  millions, 
it  has  a  diameter  about  one-third  of  that  of  the  earth. 
It  can  be  seen  at  certain  times  just  after  sunset,  and  at 
others  just  before  sunrise,  as  it  never  quits  the  neigh 
bourhood  of  the  sun.  It  is  eighty-four  days  in  travers- 
ing its  orbit,  so  that  its  year  is  less  than  a  quarter  of 
ours.  Its  orbit  is  represented  in  Plate  II.,  and,  like 
the  moon's,  is  slightly  inclined  to  the  plane  of  the 
ecliptic  ;  that  is  to  say,  if  the  earth's  orbit  is  supposed 
to  be  floating  on  the  surface  of  water,  part  of  Mercury's 
orbit  would  be  slightly  below  the  surface  and  part 
over.  From  the  diagram  you  will  see  that  Mercury 


ASTRONOMY.  63 


will  always  appear  to  us  near  the  sun.  When  it  is 
on  our  left  of  the  sun  it  apparently  follows  the  sun  on 
its  daily  course,  and  sets  just  after  it ;  when  on  the 
other  side  it  precedes  the  sun,  and  therefore  sets  before 
it,  and  so  is  only  seen  in  the  morning,  when  it  rises 
just  before  the  sun. 

151.  If  Mercury  be  watched  with  a  telescope  it  is 
found  to  go  through  the  same  changes  as  our  moon, 
and  for  the  same  reason.     You  will  understand  this 
from  Fig.  28,  where  the  ball  may  be  taken  to  represent 
Mercury  in  its  different  positions  as  it  revolves  in  its 
orbit.      When  it  is  between  us  and  the  sun  (or  in 
what  is  called  inferior  conjunction)   we  do  not 
see  it  as  its  dark  side  is  turned  towards  us,  and  as 
it  moves  round  we  see  more  and  more  of  the  bright 
side,  till  when  it  is  opposite  to  us,  or  in  what  is  called 
superior  conjunction,  we  see  the  whole  of  the 
bright  side. 

152.  Little  is  known  of  Mercury  itself;  we  know 
not  whether  it  has  a  land  and  water  surface  like  the 
earth   or  is   waterless   like  the  moon,  whether  it  is 
enveloped  in  a  dense  cloudy  atmosphere  which  pro- 
tects  the    inhabitants,   if  such  there  be,    from    the 
intense  heat  of  the  sun,  or  not.     We  only  know  that 
its  density  (Art.  133)  is  greater  than  that  of  the  earth. 

VENUS.  ' 

153.  Next  to  Mercury  comes  Venus,  at  about  66 
millions  of  miles  from  the  sun,  with  a  diameter  nearly  as 
large  as  the  earth.     It  can  generally  be  seen  either  just 
after  sunset  or  before  sunrise,  according  to  its  position 
in  its  orbit  round  the  sun,  in  the  same  manner  as 
Mercury,  only  its  orbit  being  outside  that  of  Mercury 


64 


SCIENCE  PRIMERS. 


[§iv. 


it  can  get  further  away  from  the  sun's  apparent  place 
among  the  stars,  consequently  we  can  examine  it  better. 
It  is  the  brightest  of  the  planets,  and  when  visible 
cannot  be  mistaken.  It  takes  224  days  to  perform  its 
annual  revolution,  and  23  hours  and  a  quarter  for  its 
rotation  on  its  axis,  which  determines  the  length  of 
its  day. 

154.  We  have  shown  in  speaking  of  the  earth  that 
the  inclination  of  its  axis  produces  the  seasons,  and 
that  the  pole  of  the  earth,  instead  of  being  upright  or 
perpendicular  to  the  ecliptic,  is  inclined  23°  (Art.  71). 

In  the  case  of 
Venus  there  is 
affirmed  to  be  an 
inclination  of  50°, 
or  about  half-way 
between  upright 
and  horizontal  ; 
the  consequence 
is  that  the  seasons 
there  change  to 
a  much  greater 
extent  than  ours 
do. 

154.  Venus  also 
goes  through  the 
sime  change  of 
phases  as  Mer- 
cury does,  and 

FIG.  30.— Venus,  showing  the  markings  on  its        of        COUrSC        lor 

the  same  reason. 

Very  little  is  known  of  the  surface'  of  Venus  :  certain 
dark  markings,  however,  are  seen  frequently  with  first- 


ASTRONOMY. 


rate  instruments  on  the  surface,  which  may  possibly 
be  breaks  in  clouds,  through  which  the  planet  itself  is 
seen.  The  density  of  Venus  is  about  the  same  as 
that  of  the  Earth. 

155.  If  you  will  think  a  little  you  will  see  that  in  the 
case  of  Venus  the  apparent  size  as  seen  from  the  earth 
should  greatly  change,  as  the  nearer  she  is  to  us  the 
larger  would  she  be  if  we  could  see  her  completely;  so 
that,  although  like  the  moon  she  has  phases,  unlike 
the  moon  her  size  will  alter.  Let  us  inquire  into  this 
a  little  closer.  When  Venus  is  nearly  between  us 
and  the  sun — when,  therefore,  we  can  only  see  a  fine 
crescent — she  will  be  but  some  25  millions  of  miles 
away  from  us  (because  we  are  91  and  she  66  millions  of 


FIG.  31.— Apparent  size  of  Venup.  at  its  least,  mean,  and  greatest  distance 
from  the  Earth. 

miles  from  the  sun) ;  but  when  she  is  on  the  other  side 
of  the  sun  she  will  be  157    millions  away  from  us 


66  SCIENCE  PRIMERS.  [§  v. 

(that  is,  91  millions  from  us  to  the  sun  and  66  millions 
from  the  sun  to  Venus  on  the  other  side),  so  that  her 
size  will  vary  in  the  proportion  of  157  to  25,  or  say  6 
to  i ;  so  that  the  crescent  of  Venus  will  appear  to  form 
part  of  a  circle  6  times  larger  than  that  presented  by 
Venus  when  she  is  full  to  us.  These  changes  are 
shown  in  Fig.  31. 

156.  Venus  and  Mercury,  at  times  when  they  are 
on  the  earth's  side  of  the  sun,  are  visible  as  black 
spots  on  the  sun's  disc.     This  is  called  a  transit  of 
Mercury  or  Venus  ;    that  is,  the  passage  of  the 
planet  exactly  between  us  and  the  sun,  so  that  it  is 
seen  on  the  sun's  disc. 

157.  A  transit  of  an  interior  planet,  like  an  eclipse 
of  the  sun  by  the  moon,  can  only  happen  when  the 
planet  passes  the  sun  at  the  time  it  is  near  one  of 
its  nodes,  that  is  when  it  passes  from  one  side  of  the 
plane  of  the  ecliptic  to  the  other.     A  transit,  in  other 
words,  can  only  happen  on  the  coincidence  of  the 
earth  and  planet  both  being  in  a  line  with  each  other 
at  either  node.     A  transit  of  Venus  happens  in  1874, 
and  again  in  1882,  and  not  again  for  105^  years. 


158.  Next  to  Venus  comes  the  Earth,  the  planet 
on    which  we   dwell,   and  which   has  already  been 
described.     We   therefore    pass   on  to   the    exterior 
planets. 

§  V.— THE  EXTERIOR  PLANETS. 

159.  The   next  member  of   our  system  is  Mars. 
Mars  revolves  in  an  orbit  having  a  mean  or  average 
distance  of  139  millions  of  miles  from  the  sun.     It 


ASTRONOMY.  67 


revolves  on  its  own  axis  in  24  hours  and  a  half, 
making  its  days  half  an  hour  longer  than  ours.  Its 
diameter  is  about  one  half  that  of  our  earth. 

1 60.  Mars  requires  686  days  to  complete  its  annual 
revolution    round   the    sun,    making   its    year  nearly 
double  the  length  of  ours.     Since  its  orbit  lies  outside 
ours  this  planet  never  can  pass  between  us  and  the  sun, 
mid  consequently  it  does  not  show  the  same  phases  as 
Venus  or  Mercury ;  it  however  at  two  positions  in  its 
orbit  becomes  what  is  called  gibbous,  losing  appa- 
rently its  brightness  to  a  small  extent  on  one  side,  as 
will  be  seen  in  Fig.    29,   where  the  two   positions, 
when  the  earth  is  at  J£,  are  marked  A  and  B,  and 
at  these  two  points  a  small  part  of  the  dark  side  will 
be  turned  towards  us,  presenting  an  appearance  like 
the  moon  two  or  three  days  before  or  after  full. 

1 6 1.  When  Mars  is  on  the  opposite  side  of  us  to 
the  sun  at  M,  it  is  said  to  be  in  opposition ;  it  is  then 
at  its  nearest  point  to  us  (its  distance  being  139  —  91 
=  .48  millions  of  miles)  and  fully  illuminated  ;*so  then 
this  is  the  time  to  examine  the  planet.     Its  orbit  is, 
however,  very  eccentric  or  oval,  consequently  it  is 
much  nearer  the  earth's  orbit  in  one  direction  than 
in  others  ;  and  when  an  opposition  happens,  as  is  the 
case  when  Mars  and  the  Earth  are  in  this  position  of 
their  orbits  closest  together,  we  have  a  most  favour- 
able opposition,  at  which  time  Mars  is  only  about  half 
the  distance  it  is  from  us  at  the  most  unfavourable 
one.     The  inclination  of  its  axis  is  nearly  the  same 
as  that  of   the  earth,  being  about  29°,  so  that   the 
Martial  seasons  must  be  very  similar  to  ours. 

162.  When  looked   at  with  the  eye  alone,   Mars 
appears  of  a  reddish  tint,  by  which  it  can  be  easily 


63  SCIENCE  PRIMERS.  [§  v. 

recognized,  but  when  seen  through  a  telescope  the 
redness  in  a  measure  disappears,  and  the  planet 
appears  to  have  a  bright  surface,  on  which  are  darker 
portions,  the  former  being  the  lands,  and  the  latter 
the  seas.  Mars  is  the  most  remarkable  among  the 
planets  in  this,  that  it  appears  to  us  as  the  earth 
would  appear  to  its  inhabitants.  Around  the  poles 
the  surface  appears  white,  and  on  watching  the  spots 


FIG.  32. — Mars,  showing  snow  cap  at  the  pole,  and  the  lands  and  seas. 

from  time  to  time  each  is  seen  to  grow  small  as 
summer  is  approached  in  that  hemisphere  while  the 
.opposite  one  gets  larger  in  winter,  so  we  suppose 
these  to  be  the  polar  snows  corresponding  to  those 
on  our  earth.  The  drawing  will  give  some  idea  of 
the  appearance  of  Mars  as  seen  in  a  large  telescope, 


ASTRONOMY. 


69 


one  of  the  main  features  being  that  instead  of  there 
being  about  four  times  more  water  than  land  as  on 
our  earth,  there  is  on  Mars  about  four  times  moie 
land  than  water. 

THE  ASTEROIDS. 

163.  Beyond  Mars  we  come  to  the  Asteroids,  or 
minor  planets,  a  number  of  small  bodies  not  varying 


FIG.  33. — Mars.    View  of  another  part  of  the  planet. 

greatly  in  distance  from  the  sun,  and  revolving  in 
orbits  outside  that  of  Mars.  Vesta,  Juno,  Ceres,  and 
Pallas  are  the  principal  ones,  but  they  are  only  some 
few  hundred  miles  in  diameter,  and  are  barely  visible 
to  the  naked  eye,  if  at  all,  and  from  their  smallnessare 
worth  little  notice.  Their  orbits  are  more  inclined  to 


70  SCIENCE  PRIMERS.  [§  v. 

the  plane  of  the  ecliptic  than  those  of  the  larger 
planets,  but  we  have  no  knowledge  of  the  inclination 
of  the  poles  of  these  small  planets  to  their  orbits. 
Their  number  is  large,  about  130  ;  and  we  say  about, 
for  several  are  discovered  every  year,  and  the  names 
of  nearly  all  the  deities  must  have  been  used  for 
them.  The  greater  number  of  these  are  only  equal 
to  a  loth  magnitude  star  in  brilliancy,  and  their 
surface  may  possibly  be  not  much  larger  than  the  area 
of  a  good  Scotch  estate. 

JUPITER. 

164.  Outside  the  orbits  of  the  numerous  asteroids  is 
the  largest  planet  of  our  system,  Jupiter,  a  body  that  has 
no  doubt  been  pointed  out  to  you  some  time  or  other. 
When  above  the  horizon,  it   is   unmistakable  by  its 
excessive  brightness,  being  only  surpassed  by  Venus, 
which    can   generally  be  recognized  from   it   by  its 
proximity  to  the  sun.     Jupiter  revolves  in  an  orbit  at 
a  distance  of  476  millions  of  miles  from  the  sun,  com- 
pleting his  year  in  4,333  days. 

165.  When  observed  with  a  telescope  of  moderate 
power,  Jupiter  appears  of  an  oval  shape,  very  much 
flattened  at  the  poles,  and  crossed  by  several  dark 
belts,  as  represented  in  the  figure  ;  large  black  spots  and 
other  markings  of  which  we  shall  say  more  presently, 
are  also  frequently  seen  on   the   surface,  and  from 
the  motion  of  those  markings,  the  time  of  rotation  on 
its  axis  has  been  ascertained  to  be  about  10  hours, 
that  is  less  than  half  one  of  our  days,  and  its  dia- 
meter is  found  to  be  about  ten  times  the  diameter  of 
our  earth,  so  that  the  flattening  of  the  poles  and  the 


ASTRONOMY.  71 


protuberance  of  the  equator  must  necessarily  greatly 
exceed  that  of  our  earth,  for  the  velocity  that  the 
equator  moves  at  must  be  twenty  times  the  velocity  of 
our  planet  at  the  equator,  or  20,000  miles  per  hour. 

1 66.  We  have  mentioned  the  belts  and  other  mark- 
ings on  its  surface;  it  is  probable  that  Jupiter  is  covered 
with  clouds,  giving  rise  to  its  bright  appearance,  and 
that  the  dark  belts  are  openings  in  the  clouds  through 


FIG.  34.— Jupiter,  showing  the  cloud  belts. 

which  we  see  the  darker  surface  of  the  planet,  or 
more  probably  of  lower  beds  of  clouds  beneath. 
The  number  and  size  of  the  belts  are  continually 
changing,  and  bridges  of  cloud  are  constantly  being 
thrown  over  the  dark  spaces,  clearly  showing  that  it 
is  not  the  surface  of  the  planet  we  see,  but  only  a 
very  cloudy  atmosphere. 
8 


SCIENCE  PRIMERS. 


[§v. 


167.  So  far  as  we  have  gone  the  planets  have  been 
unlike  the  earth  in  one  respect,  they  have  no  moons. 
Jupiter,  however,  has  four  satellites  or  moons  revolving 
round  him,  and  going  through  the  same  changes  as 
our  own.  They  are  all  nearly  of  the  same  size,  about 
2.000  miles  in  diameter,  but  at  different  distances, 
and  consequently  they  take  very  different  times  to 
revolve  round  their  primary,  Jupiter,  the  first  taking 
less  than  2  days,  the  second  3^  days,  the  third  7 


FIG.  35. —  Diagram  explaining  the  eclipses,   occultations,  and  transits  of 
Jupiter's  satellites. 

days  3  hours,  the  fourth  i6j  days.  They  all  re- 
volve in  orbits  very  slightly  inclined  to  the  plane  of 
Jupiter's  orbit,  and  consequently  whenever  they  pass 
between  the  sun  and  Jupiter  there  is  an  eclipse  of  the 
sun  visible  on  some  part  or  other  of  the  planet's 
surface ;  only  the  fourth  has  an  orbit  sufficiently 
inclined  to  enable  it  to  pass  above  or  below  the  line 
joining  the  sun  and  Jupiter,  this  prevents  it  from 
causing  an  eclipse  at  every  resolution.  For  the  same 


ASTRONOMY.  73 


reason  of  course  the  moons  also  are  eclipsed  at  every 
revolution  by  the  planet's  shadow. 

1 68.  When  viewed  with  a  telescope  the  moons 
appear  to  oscillate  on  either  side  of  Jupiter  (just  as 
the  interior  planets  appear  to  us  to  oscillate  on  either 
side  of  the  sun),  and  in  their  passage  from  one  side 
to  the  other  they  generally  pass  over  the  disc  of  the 
planet;  there  is  then  what  is  called  a  " transit"  of 
the  moon  over  the  disc.  We  also  see  the  shadow  of 
the  moon  traversing  the  disc  whenever  we  are  so  far 
from  the  line  joining  the  sun  and  Jupiter,  that  the  moon 
does  not  cover  the  shadow.  The  moons  in  passing 
round  on  the  other  side  at  times  suddenly  disappear, 
or  are  eclipsed,  when  they  pass  into  the  shadow 
of  the  planet,  but  we  may  be  in  such  a  position  that 
Jupiter's  shadow  lies  on  the  opposite  side  of  the  planet 
to  that  behind  which  the  moon  passes  ;  the  satellite 
chengoes  behind  the  disc  uneclipsed,  and  is  said  to  be 
"occulted."  The  diagram  will  make  this  clearer; 
when  the  earth  is  at  the  point  E  of  its  orbit,  the  moon 
/^appears  in  transit,  while  the  Mis  occulted  and  O 
eclipsed,  and  from  this  point  of  view  every  satellite 
must  be  occulted  before  it  is  eclipsed ;  but  when  the 
earth  is  at  /^the  moon  J/is  no  longer  occulted,  and 
will  pass  into  the  shadow  and  become  eclipsed 
without  an  occultation,  and  from  this  point  P  will 
be  in  transit  and  O  also  eclipsed,  but  as  soon  as  it 
leaves  the  shadow  it  will  be  behind  the  planet,  and 
will  reappear  from  an  occultation. 

169.  The  inclination  of  Jupiter's  axis  is  very  small, 
only  a  little  over  4°,  so  that  there  can  be  no  appreci- 
able change  in  the  Jovian  seasons.  Although  the  size, 
or,  more  correctly  speaking,  the  volume,  of  Jupiter  is 


74 


SCIENCE  PRIMERS. 


V, 


more  than  1,300  times  that  of  the  earth, — that  is,  1,300 
globes  of  the  size  of  our  earth,  if  made  into  one 
world,  would  only  be  of  the  size  of  Jupiter, — still  its 
weight  is  only  300  times  the  weight  of  the  earth,  so 
that  the  materials  composing  Jupiter  are  of  a  much 
lighter  kind  than  those  composing  the  earth  ;  thus 
representing  the  density  of  the  earth  by  i,  Jupiter's 
density  is  less  than  \. 

SATURN. 

170.  We  next  come  to  Saturn,  a  truly  grand  sight  in 
a  telescope,  Saturn  having,  besides  eight  moons,  an 
immense  bright  ring  surrounding  the  globe.  This 


FIG.  36. — Saturn  and  his  rings. 

planet  revolves  in  an  orbit  at  about  872  millions  of  miles 
from  the  sun,  taking  10,759  days,  or  nearly  thirty  of  our 
years,  to  complete  its  year,  and  having  a  diameter  nine 


ASTRONOMV.  75 


times  greater  than  that  of  the  earth.  From  observa- 
tions of  spots  and  belts  on  the  surface  (somewhat 
similar  to  those  on  Jupiter)  the  time  of  its  diurnal 
revolution  has  been  fixed  at  about  10^  hours,  a  little 
longer  than  that  of  Jupiter,  and  it  is  probable  that 
Saturn  has  much  the  same  constitution  as  that  planet, 
as  it  appears  to  us  to  be  covered  with  an  extensive 
cloudy  atmosphere  producing  belts  as  on  Jupiter;  it 
is  also  made  up  of  very  much  lighter  materials  than 
our  earth  is,  materials  of  only  half  the  density  of  those 
composing  Jupiter.  Saturn's  axis  is  inclined  at  an 
angle  of  about  265°,  so  there  are  seasons  there  as  on 
our  earth. 

171.  Now  as  to  the  rings,  what  are  they  ?     Their 
general  appearance  is  that  of  three  rings  lying  outside 
each  other  in   succession  as  shown  in  the  diagram, 
Fig-  36,  the  diameter  of  the  outer  ring  being  about 
166,000  miles.     The  two  outer  ones  are  the  brightest, 
the   inner   or  crape  ring  being  only  just  visible  in 
a  large  telescope,  the  ball  of  the  planet  being  seen 
through  it.     In  spite  of  their  enormous  breadth,  the 
thickness  of  the  rings  is  only  about  138  miles,  and 
when  edgeways  to  us,  as  is  the  case  in  certain  positions, 
when  Saturn  moves  in  its  orbit,  they  are  barely  visible 
iti  the  best  telescopes.     It  is  thought  that  the  rings 
represent   a  vast   assemblage   of  small   satellites   or 
moons  revolving  round  Saturn. 

172.  The  moons  of  Saturn,  eight  in  number,  are  not 
of  such  interest  as  those  of  Jupiter.     Their  distance 
from  us  precludes  us  generally  from  observing  their 
eclipses  and  occultations  ;  their  orbits  also  are  largely 
inclined  to  the  orbit   of    Saturn,   and    consequently 
eclipses  are  rare. 


76  SCIENCE  PRIMERS. 


URANUS. 

173.  We  next  come  to  Uranus,  of  which  little  is 
known,  its  distance — 1,753  millions  of  miles  from  the 
sun,  being  so  immense ;  it  takes  30,686  of  our  days  to 
complete  its  annual  revolution,  and  it  is  known  to 
have  four  moons.     Its  diameter  is  four  times  greater 
than  that  of' our  earth,   and    its   density  is  about  \ 
that  of  the  earth. 

/ 

NEPTUNE. 

174.  Then  comes  Neptune,  the  most  distant  planet 
of  our  system  at  present  known,  at  2,746  millions  ot 
miles  from  the  sun,  and  taking  60,126  days  to  go  round 
the  sun.     Its  diameter  is  over  four  times  greater  than 
that  of  our  earth,  and  its  density  is  slightly  less  than 
that  of  Uranus. 

175.  Its  discovery  is  interesting  as  showing  how  the 
position,  mass,  and  other  attributes   of  a  planet  can 
be  calculated  by  their  effect  on  other  bodies  at  a  dis- 
tance before  the  planet  has  actually  been  seen.     It 
had  been  noticed  for  a  long  time  that  Uranus  moved 
at  one  part  of  its  orbit  slower,  and  at  another,  faster, 
than  its  proper   rate,    and   from   these   observations 
the  position,  mass,  period,  &c.  of  the  planet  were  de- 
termined before  it  had  ever  been  seen,  and  it  was  found 
very  close  indeed  to  its  calculated  place.      Neptune 
has  only  one  moon  at  present  discovered. 


ASTRONOMY. 


77 


§  VI.—  COMETS,  METEORITES,  AND  FALLING 
STARS. 

176.  Besides  the  planets,  there  are  other  members 
of  our  system,  of  a  different  kind.     We  may  say  that 
the  planets  are  the  members  of  the  solar  household  ; 
the  bodies  we  are  about  to  consider  are  visitors. 

177.  Those  who  have  seen  a  comet  will  not  require 
to  be  reminded  of  the  strange  appearance  of  those 
bodies,  and  those  who 

have  not  seen  one  will 
get  some  idea  of  what 
this  class  of  bodies  is 
like  from  the  diagram. 
Comets  vary  so  much  in 
form  a.nd  size  and  bright- 
ness, that  no  two  are 
precisely  alike :  some- 
times they  resemble  a 
small  planet  or  star  with 
a  bright  point  called 
the  nucleus,  an  im- 
mense tail  stretching  for 
millions  of  miles  behind; 
at  other  times  they  ap- 
pear with  a  nucleus  with 
mist  extending  equally 

round    it  ;     in    fact,    their         FlG'  37-General  view  of  a  Comet. 

shapes  are  almost  as  various  as  those  of  the  clouds. 
The  greater  number  of  comets  are  invisible  to  the 
naked  eye. 

178.  The  majority  of  comets  that  come  into  our 
system  from  outside,  are  attracted  towards  the  sun,  pass 


73  SCIENCE  PRIMERS.  [§  vi. 

by  it,  and  then  continue  on  away  from  our  system 
again;  while  there  are  others  that  belong  to  our 
system,  and  revolve  round  the  sun  as  the  planets  do, 
only  instead  of  having  nearly  circular  orbits,  their 
paths  are  very  eccentric,  so  that  the  comets  approach 
near  the  sun  at  one  time,  and  then  recede  to  immense 
distances  away.  There  are  several  such  comets  whose 
orbits  are  known,  and  these  are  called  after  their  dis- 
coverers;  such  as  Encke's  comet,  which  revolves 
round  the  sun  once  every  five  years,  and  Halley's, 
that  has  a  period  of  about  seventy-four  years. 

179.  The  orbits  of  comets  have  very  various,  and 
some  of  them  very  great,  inclinations,  not  like   the 
orbits  of  planets,  which  all  lie  nearly  in  the  same 
plane,    the  plane  of  the   ecliptic ;    the   majority  go 
round  the  sun  the  contrary  way  to  planets,  and  are 
said  to  have  a  retrograde  motion. 

1 80.  Their  weight  is  excessively  small,  while  their 
volume  or  bulk  is  immense — that  of  Donati,  figured 
in  the  diagram,  having  a  tail  millions  of  miles  long, 
through  which  faint  stars,  which  a  thin  cloud  or  puff  of 
smoke  would  obscure,  were  visible.     As  a  comet  ap- 
proaches the  sun,  envelopes  or  jets  are  formed. 

1 8 1.  Now,  before  I  say  anything  more  about  these 
strange  things,  I  must  remind  you  that  perhaps  when 
you  have   been  looking  at  the  sky,  you  may   have 
noticed   a  bright   point,   like   a   star,    shoot   rapidly 
across   the  heavens,   leaving  a  bright   streak  for  a 
second  or  two  behind  it.     Several  may  generally  be 
seen  every  fine  night  with  a  little  attention.     These 
are  called  meteors   or    falling  stars,  or,  if  they 
actually  fall,  as  some  do,  to  the  earth,  meteorites. 
They  vary  greatly  in  apparent   size  and  brightness 


ASTRONOMY. 


79 


the  smaller  being  most  prevalent;  the  larger,  called 
meteors,  are  rare,  and  sometimes  appear  as  large 
and  almost  as  bright  as  Jupiter  or  the  Moon,  and 
traverse  the  sky  for  some  seconds,  leaving  a  luminous 
trail  behind  them. 

182.  Now  of  course,  as  some  of  these  bodies  fall  to 
the  earth,  the  chemist  can  examine  them  and  find  out 
what  they  are  made  of,  as 

he  has  found  out  what  the 
earth  is  made  of.  Some 
are  especially  metallic  in 
their  nature,  others  espe- 
cially stony.  As  they  rush 
into  our  atmosphere  they 
are  heated  so  hot  that 
they  burn,  and  the  small 
ones  are  consumed  before 
they  can  reach  the  earth  ; 
the  larger,  on  the  other 
hand,  are  not  entirely  con- 
sumed, though  melted  on 
the  surface  and  consider- 
ably reduced  in  size.  A 
number  of  these  that  have 
escaped  destruction  are  to 
be  seen  in  the  British  Comet. 

Museum,  some  reaching  the  weight  of  three  tons. 

183.  From  constant  observation  it  has  been  found 
that  on  differents  nights  the  majority  of  shooting  stars 
appear  to  come  irom  certain  parts  of  the  sky,  and 
on  certain  nights  in  the  year  many  more  fall  than  on 
others.     There  are,  for  instance,  the  well-knoAvn  falls 
of  November  13  and  August  10,  those  of  November 


FIG.  38. — Head  and  envelopes  of 


8o  SCIENCE  PRIMERS.  [§  vi. 

coming  from  the  constellation  Leo,  and  consequently 
called  the  Leonides,  and  those  of  August  from  Perseus, 
and  called  the  Perseids. 

184.  We  now  know  that  these  meteors  travel  round 
the  sun  as  the  planets  do,  and  the  strange  thing  is 
that  when  we  come  to  examine  the  shape,  size,  and 
position  of  their  orbits,  they  are  found  to  be  the  same 
as  those  of  some  of  the  comets;  so  that  since  some  me- 
teorites and  comets  have  the  same  path  or  orbit,  it  has 
been  suggested  that  comets  are  clouds  of  meteorites. 
This  hint  of  a  connection  between  comets  and  meteor- 
ites is  one  of  the  greatest  discoveries  of  late  years  in 
the  science  of  astronomy ;  and  the  observations  on  the 
beautiful   comet  visible   in    1874   have    shown   thai 
possibly  the  heat  and  light  of  a  comet  may  be  due 
to  the  clashing  together  in  space  of  these  very  bodies 
which,  when  they  fall  into  our  air,'  give  rise  to  the 
appearance  of  falling  stars,  for  we  know  that  comets 
are  not  very  hot,  that  they  do  partly  consist  of  solid 
particles  or  masses,  and  that  the  vapour  given  off  is 
that  of  a  substance  known  to  exist  in  meteorites. 

185.  Comets,  from  their  sudden  and  curious  appear- 
ance, were  looked  on  with  great  awe  by  the  ancients, 
and  all  kinds  of  calamities  were  attributed  to  them.  We 
learn,  for  instance,  that  about  the  year  975  the  Ethio- 
pians and  Egyptians  felt  the  dire  effects  of  the  comet 
to  which  Typhon,  who  reigned  then,  gave  his  name. 
It  appeared  all  on  fire,  and  was  twisted  in  the  form  of 
a  spiral,  and  had  a  hideous  aspect.     It  was  not  so 
much  a  star  as  a  knot  of  fire.     We  thus  see  how 
science  replaces  the  terror  felt  in  past  ages  by  an 
admiration  of  the  wonders  of  the  universe  in  which 
we  dwell 


ASTRONOMY.  81 


IV.— THE  SUN— THE  NEAREST  STAR. 

§  I.— THE  INFLUENCE   OF   THE    SUN   IN   THE 
SOLAR  SYSTEM. 

1 86.  In  what  has  gone  before  I  have  tried  to  show 
you  what  the   Earth  is — (I  do  not  mean  what  it  is 
made  of;  that  you  will  learn  in  the  Chemistry  Primer  : 
or  what  it  is  like — how  .its  surface  is  one  of  land  and 
sea,  or  how  it  is  surrounded  by  an  atmosphere — that 
you  will  learn  in  the  Physical  Geography  Primer) — and 
we  have  found  that  it  is  a  cool  body  travelling  round 
the  sun,  and  because  itj  is  cool  it  has  no  light  of  its 
own,  its  light  being,  as  a  matter  of  fact,  borrowed 
from  the  sun. 

187.  Next,  I  have  shown  you  that  it  is  one  of  several 
similar  bodies  travelling  round  the  sun,  which  bodies, 
called  planets,  are  cool  like  the  earth,  and  as  such 
they  give  out  no  light  of  their  own. 

1 88.  We  have  also  seen  that  the  length  of  the  earth's 
year,  and  of  the  years  of  the  other  planets,  depends 
upon  the  time  each  planet  takes  to  go  round  the  sun ; 
and  further,  that  the  length  of  the  earth's  day,  and  of 
the  days  of  the  other  planets,  depends  upon  the  rate 
at  which  each  planet  spins  round,  and  so  brings  each 
part  of  its  surface  into  the  sunlight. 

189.  Further,  we  have  seen  how  the  inclination  of 
the  axis  of  the  earth,  and  of  that  of  each  planet,  de- 
termines the  seasons,  the  change  of  which  is  chiefly 
due  to  the  difference,  at  any  one  period  of  the  year, 
between  the  time  during  which  each  part  of  a  planet 
is  exposed  to  the  sun  and  the  time  during  which  it 
is  withdrawn  from  the  sun's  influence. 


82  SCIENCE  PRIMERS.  [§  n. 

190.  So  that  you  see  the  sun  has  to  do  with  every- 
thing. What,  then,  is  this  Sun,  which  occupies  the 
central  position  round  which  all  the  planets  travel, 
and  which  is  so  important  to  them  that  their  very  life 
as  it  were  depends  upon  its  rays  ? 


§  II.— THE  HEAT,  LIGHT,  SIZE  AND  DISTANCE 
OF  THE  SUN. 

191.  First,  I  have  to  tell  you  that  you  may  regard  the 
sun  as  a  globe  of  the  fiercest  fire  :  the  heat  of  the  sun 
is  so  enormous  that  it  is  useless  for  me  to  attempt  to 
give  you  any  idea  of  it.     Remember,  I  have  already 
told  you  that  the  other  planets,  like  the  earth,  are 
cool  bodies ;  that  is,  bodies  on  the  surface  of  which 
various  substances  can  exist  in  the  solid  state  :  hence 
we  talk  of  the  "solid  earth."     But  on  the  sun  nothing 
is  solid,  everything  exists  in  the  shape  of  white  hot 
vapour. 

192.  Next,  I  have  to  tell  you  that  in  consequence 
of  this  tremendous  heat,  the  sun  shines  by  its  own 
light.     Remember,  I  have  told  you  that  the  planets 
and  their  moons  (including  of  course  our  moon)  do 
not. 

193.  And  lastly,  I  have  to  tell  you  that  the  sun  is  a 
globe  of  such  enormous  dimensions,   that  it  is   500 
times  larger  than  all  the  planets  put  together.     If  you 
were  to  take  nearly  \\  millions  of  Earths,  and  knead 
them  into  a  ball,  you  would  then  have  a  globe  about 
as  large  as  the  sun. 

194.  I  have  already  told  you  that  the  distance  of 
the  sun  from  us  is  about  91  millions  of  miles.      To 


ASTRONOMY.  83 


go  into  the  mode  of  measurement  would  lead  us  too 
far  into  mathematics  for  my  present  purpose ;  but  it 
may  be  stated  here  that  knowing  its  distance  and 
apparent  size,  we  can  proceed  to  find  its  diameter  in 
this  way.  Let  us  draw  imaginary  lines  from  either 
side  of  the  sun  to  the  eye,  as  AB  and  AC,  Fig.  39, 

C 


FIG.  39. — How  the  size  of  ths  Sun  is  determined. 

CB  representing  the  diameter  of  the  sun,  we  find  that 
the  inclination  of  the  two  lines  to  each  other  is  such 
that  all  lines  drawn  from  one  line  to  the  other,  as 
DE  or  FG,  are  equal  in  length  to  T^T  of  their  dis- 
tance from  A,  so  also  B  C  is  T£T  part  of  the  distance 
A  B,  which  we  know  is  91  millions  of  miles;  divid- 
ing this  by  107  we  get  850,467,  which  is  the  distance 
from  B  to  C,  or  the  diameter  of  the  sun  in  miles. 

§  III.— WHAT  THE  SUN  IS  LIKE. 

195.  There  are  not  many  observations  that  can  be 
made  on  the  sun  without  the  aid  of  a  telescope  and 
dark  glasses,  and  its  intense  heat  and  light  render  it 
dangerous  to  look  at  it  without  special  precautions.1  If 
you  smoke  a  piece  of  glass  over  a  candle,  and  look  at 
the  sun  through  it,  it  will  appear  to  be  a  round  bright 
object,  because  each  part  of  it  shines  by  its  own  light : 
unlike  the  moon,  it  is  always  round.  This  bright 
part  is  called  the  photosphere.  In  telescopes 

*  The  young  reader  must  not  attempt  to  look  at  the  sun  through  a  small  tele- 
scope, for  he  or  she  may  be  blinded  in  the  attempt. 
9 


84  SCIENCE  PRIMERS.  [§  iv. 

black  spots  are  frequently  seen  on  its  surface,  and 
these,  indeed,  are  sometimes  of  sufficient  size  to  be 
visible  without  the  telescope. 

196.  In  the  neighbourhood  of  the  spots  brighter 
portions  than   the  general    surface   are   seen  :   these 
are  called  faculse,  and  probably  are  immense  banks 
of  brighter  vapours  several  thousands  of  miles  long. 
If  the  spots  and  faculse  be  watched  from  time  to  time 
they  will  be  found  to  be  constantly  changing  their 
shape- 

§  IV.-SUN-SPOTS. 

197.  Although  the  sun   is  so  far  away  from  us, 
in  consequence  of  its  immense  size  and  the  violence 
of  the  forces  at  work,  these  spots  are  fine  objects  in 
the  telescope.     I  give  a  drawing  of  one  (Fig.  40)  so 
large   that  several  Earths   might  have   been   hurled 
into  it. 

198.  If  these  spots  be  observed  and  their  positions 
carefully  noted,  and  again  observed  one  or  two  days 
afterwards,  they  will  be  found  to  have  changed  their 
position  towards  the  west,  and  they  will  be  seen  to  be 
gradually  moving  from  the  east  side  of  the  sun's  disc 
to  the  west,  where  they  will  gradually  disappear. 

199.  Now,  since  all  of  these  have  the  same  motion 
in  the  same  direction,  it  is  evident  that  the  surface 
of  the  sun  is  moving  and  carrying  the  spots  with  it, 
and  if  a  well-marked  spot  be  observed  when  passing 
off  the  disc  to  the  west,  it  will  be  found  about  12  days 
after  to  appear  again  on  the  east  side  and  get  to  the 
osition  where  it  was  first  observed  in    about  25  days, 


ASTRONOMY. 


having  in  that  time  gone  right  across  the  disc  and 
round  the  back. 

200.  The  surface  of  the  sun  has  therefore  moved 
round  in  25  days,  or  in  reality  the  whole  sun  itself 
is  turning  round  on  its  axis  at  this  rate,  carrying  spots 
and  faculse  with  it. 

20 1.  Let  us  now  see  what  kind  of  thing  a  spot  is. 
If  a  pretty  regular  one  is  observed  near  the  middle  of 


FJG.  40.— A  Sun-spot. 

the  disc  it  appears  round  ;  if  it  be  again  observed  a  few 
days  after,  near  the  edge,  it  will  appear  no  longer  of  the 
same  shape,  the  darkest  middle  part  having  apparently 
moved  to  the  left  while  the  half  shade  round  it  has 
vanished.  Let  us  see  what  we  can  learn  from  this. 
Take  an  ordinary  saucer,  and  having  blackened  the 
part  of  it  on  which  the  cup  generally  stands,  look 


86 


SCIENCE  PRIMERS. 


straight  at  it — you  will  see  the  black  part  equally  sur- 
rounded by  the  sloping  sides,  as  at  A;  now  twist  the 
saucer  till  it  is  seen  more  edgewise,  and  you  will  see 
the  edge  on  the  left  hand  quite  disappear,  while  the 
right  side  is  nearly  flat  in  front  of  the  eye,  and  it  will 
have  the  appearance  of  C. 

202.  Now,  if  a  cavity  like  the  saucer  were  cut  on  a 
large  globe,  it  would  go  through  just  the  same  changes 
that  we  find  the  saucer  and  the  spot  do,  so  we  may 
conclude  that  the  spots  on  the  sun  are  hollows  in  the 


FIG.  41. — Explanation  of  the  appeara  ices  presented  by  Sun-spots. 

bright  substance  of  the  sun  but  it  is  found  from 
other  evidence  that  these  hollows  are  not  empty,  but 
filled  with  gases  stopping  the  light  given  out  below. 


§  V.— THE  SUN'S  ATMOSPHERE. 

203.  The  round  sun  that  we  see  is  not  all  there 
is  of  the  sun,  but  only  the  denser  part  of  it ;  the  less 
dense  and  luminous  vapours  extend  for  hundreds  of 
thousands  of  miles  beyond  the  visible  sun;  but 
generally  we  cannot  see  them  any  more  than  we 
can  the  stars;  still,  in  Eclipses,  when,  as  we  have 
seen,  the  light  of  the  sun  is  cut  off  by  the  moon, 
we  can  see  them,  as  we  can  see  the  stars  (Art. 
114).  The  luminous  vapours  then  appear  of  exquisite 


ASTRONOMY.  87 


colours,  red  being  most  common.  These  vapours, 
however,  get  brighter  nearer  the  sun,  and  form  an 
envelope  round  him,  called  the  Chromosphere,  and 
these  can  be  observed  by  a  special  method.  It  is 


FIG.  42. — The  Sun's  coronal  atmosphere. 

then  seen  that  the  lighter  vapours  of  the  real  sun 
are  shot  up  into  its  outer  atmosphere,  called  the 
coronal  atmosphere,  taking  fantastic  shapes  called 
prominences,  and  these  prominences  rapidly  change. 

§  VI.— WHAT  THE  SUN  IS  MADE  OF. 

204.  By  analysing  the  light  of  the  sun  by  means 
of  a  spectroscope,  an  instrument  that  splits  light  up 
into  its  component  colours,  in  the  same  manner  as 
you  have  seen  light  split  up  into  all  the  colours  of 
the  rainbow  by  the  glass  drops  on  chandeliers,  it 


88  SCIENCE  PRIMERS.  [§  I. 

has  been  found  that  a  great  number  of  our  metals 
exist  in  the  sun,  not  of  course  in  their  metallic  state, 
but  in  a  state  of  vapour,  the  heat  there  being  so 
intense  that  the  metals  evaporate  as  water  with  us 
does  into  steam.  There  are  first  of  all,  among  the 
elements  that  we  know  here,  the  gas  hydrogen,  and 
then  vapours  of  magnesium,  calcium,  sodium,  iron, 
manganese,  nickel,  barium,  strontium,  and  very  many 
more  metals,  besides  probably  two  other  gases,  not 
yet  found  on  the  earth. 

205.  Since,  as  we  have  seen,  the  sun  is  so  largely 
composed  of  gases,  you  will  not  be  surprised  that  its 
density  is  much  less  than  that  of  the  earth ;  indeed,  it 
is  less  than  a  quarter  of  that  of  our  planet. 

§  VII.— THE  SUN  IS  THE  NEAREST  STAR. 

206.  I  have  been  careful  to  dwell  at  some  length  on 
what  is  called  the  physical  constitution  of  the  sun, 
not  merely  because   in   it  we   have   an    example  of 
a  class  of  bodies  very  unlike  the  planets,  as  we  have 
seen,  but  because  we  now  know  that  the  sun  is  a 
star;  bigger  and  brighter  than  the  other  stars,  not 
because  it  is  unlike  them,  but  simply  because  it  is  so 
near  to  us.  „ 

207.  We  can  now,  then,  define  the  solar  system  to 
consist  in  the  main  of  a  number  of  cool  bodies  revolv- 
ing round  a  hot  one.     As  we  can  take  the  earth 
as  a  type  of  the  planets,  so  we  can  take  the 
sun   as   a  type   of  the  twinkling  stars  that 
people  the  depths  of  space ;  and  it  is  not  too 
much  to  believe  that  every  star  is  surrounded  by  its 
family  of  planets  in  the  same  way  as  the  sun  is. 


ASTRONOMY.  89 


V.— THE   STARS. 
^  I.-THE  STARS  ARE  DISTANT  SUNS. 

208.  From  the  sun — the  nearest  star — that  gives  u» 
heat  and  light,  we  must  now  turn  to  the  more  distant 
ones.    After  what  has  been  stated  you  will  not  be  sur- 
prised at  my  turning  from  a  large  body  like  the  sun, 
the  beams  of  which  are  so  hot,  to  those  tiny  specks 
of  light  distributed  in  the  heavens,  the  heating  power 
of  which  is  imperceptible,  since  those  little  twinkling 
bodies  are  suns,  giving  out  light  and  heat  like  our 
sun,  only  they  are  at  such  incredible  distances  from 
us, — the  distance  of  some  of  the  nearest  stars  is  more 
than   500,000  times  the  distance  of  our  sun, — that 
their  size   becomes  inappreciable :   we  have,  never- 
theless, reason  for  believing  that  many  of  them  are 
several  hundred  times  larger  than  our  sun. 

§  II.— THE  BRIGHTNESS  OF  THE  STARS. 

209.  When  we  look  at  the  stars  at  night,  one  of  the 
first  things  we  notice    is    that   they  are  of  different 
brightnesses.     Is  it  that  some  are  smaller  than  others, 
or  are  the  brightest  the  nearest  to  us  ?     It  is  difficult 
to  say  exactly,  for  in  some  cases  the  bright  stars  are 
nearest  to  us,  and  in  others  there  are  small  ones  as 
near,  so  that  both  size  and  distance  come  into  play. 

210.  Stars  are  classed  in  magnitudes  according  to 
their  order  of  brightness,  the  brightest  being  said  to 
be  of  the  first  magnitude,  the  next  of  the  second 
magnitude,  down  to  the  fifteenth  and  sixteenth,  which 
require  the   most  powerful   telescope  to  view  them. 
The  faintest  star  visible  on  a  dark  night  is  of  about 


90  SCIENCE  PRIMERS.  [§  in. 

the  sixth  magnitude.  After  what  has  been  said  you 
must  not  think  that  magnitude  means  real  size,  as  a 
large  star  may  be  far  away,  and  so  be  classed  so  far  as 
brightness  goes  with  a  smaller  one  nearer  to  us. 

211.  There  are  about  3,000  stars  from  the  first  to  the 
sixth  magnitude  visible  at  once  to  the  naked  eye,  and 
there  are  over  20,000,000  visible  in  large  telescopes. 

212.  You  may  have  also  noticed,  on  a  clear  dark  night, 
a  zone,  or  band  of  faint  light,  stretching  from  the  hori- 
zon on  one  side,  nearly  over  our  heads  to  the  horizon 
on  the  other.     This  is  called  the  milky  way.     It  is 
composed  of  an  almost  infinite  number  of  small  stars, 
apparently  so  close  together  as  to  form  a  luminous 
mass  ;  and  of  the  20.000,000  telescopic  stars,  probably 
18,000,000  are  in  the  milky  way.     A  view  of  this 
gives   us  some  little  idea  of  the  immensity  of  our 
universe,  if  we  consider  that  it  is  not  the  real  close- 
ness  of  the   stars  that  we  observe,  but   only  their 
apparent  closeness,  placed,  as  they  probably  are,  one 
almost,  behind  the  other  so  as  to  be  in  nearly  the  same 
line  of  sight,  and  at  a  distance  from  each  other  perhaps 
as  great  as  that  from  our  sun  to  the  nearest  star. 

213.  If  you  suppose  a  wood  in  which  all  the  trees 
are  the  same  distance  apart,  and  you  place  yourself  in 
the  wood  near  one  side  of  it,  the  trees  will  appear 
nearest  together  on  the  other.     So  is  it  with  the  stars 
in  the  milky  way ;  there  is  the  greatest  number  of  stars 
in  the  line  of  sight. 

214.  The  colours  of  the  stars  are  various,  some  being 
white,  others  orange,  red,  green,  and  blue.     For  in- 
stance, Sirius  is  white,   Arcturus  yellow,   Betelgeuse 
red,  but  these  colours  are  more  noticeable  with  a  tele- 
scope than  with  the  eye  alone. 


ASTRONOMY.  91 


§  III.— THE  CONSTELLATIONS. 

215.  The  stars  have  been  grouped,  as  long  as  history 
carries  us  back,  into  constellations,  each  one  of 
which  received  some  fanciful  name  according  to  the 
being  or  object  the  stars  composing  it  were  thought 
to  represent.  The  sun  in  his  course  passes  over  the 
zodiacal  constellations,  visible  of  course  both  in 
the  Northern  and  Southern  Hemispheres  of  the  Earth. 
These  are  Aries,  Taurus,  Gemini,  Cancer,  Leo,  Virgo,  Li- 
bra, Scorpio,  Sagittarius,  Capricornus,  Aquarius,  and  Pis- 
ces, the  Latin  names  for  the  Constellations,  the  order  of 
which  you  will  remember  from  the  following  rhyme  : — 

"  The  Ram,  the  Bull,  the  Heavenly  Twins, 
And  next  the  Crab,  the  Lion  shines, 

The  Virgin  and  the  Scales, 
The  Scorpion,  Archer,  and  She  Goat, 
The  Man  that  holds  the  watering-pot, 

The  Fish  with  glittering  scales." 

2i  6.  The  constellations  visible  in  the  Northern  Hemi- 
sphere above  the  zodiacal  constellations,  are  called 
the  northern  constellations,  they  are  as  follows  : 

Ursa  Major.  The  Great  Bear  (The  Plough). 

Ursa  Minor.  The  Little  Bear. 

Draco.  The  Dragon. 

Cepheus.  Cepheus. 

Bootes.  Bootes. 

Corona  Borealis.  The  Northern  Crown. 

Hercules.  Hercules. 

Lyra.  The  Lyre. 

Cygnus.  The  Swan. 

Cassiopea,  Cassiopea  (The  Lady's  Chair). 

Perseus.  Perseus. 

Auriga.  The  Waggoner. 


92 


SCIENCE  PRIMERS. 


[§iv. 


Serpenta  rius» 

Serpens. 

Sagitta. 

Aquila. 

Delphinus. 

Equuleus. 

Pegasus. 

Andromeda. 

Triangulum. 

Camelopardalis. 

Canes  Venatici. 

Vulpecula  et  Anser. 

Cor  Carol i. 


The  Serpent- Bearer. 
The  Serpent. 
The  Arrow. 
The  Eagle. 
The  Dolphin. 
The  Little  Horse. 
The  Winged  Horse. 
Andromeda. 
The  Triangle. 
The  Cameleopard. 
The  Hunting  Dogs. 
The  Fox  and  the  Goose. 
Charles'  Heart. 


217.  The  constellations  visible  in  the  Southern 
Hemisphere  above  the  zodiacal  ones,  called  the 
southern  constellations,  are : 


Cetus. 

Orion. 

Eridanus. 

Lepus. 

Can  is  Major. 

Cam's  Minor. 

Argo  Navis. 

Hydra. 

Crater. 

Corvus. 

Centaurus. 

Lupus. 

Ara. 

Corona  Australis. 

Pis cis  Australis. 

Monoceros. 

Columba  Noachi. 

Crux  Australis. 


The  Whale. 

Orion. 

The  River  Eridanub. 

The  Hare. 

The  Great  Dog. 

The  Little  Dog. 

The  Ship  Argo. 

The  Snake. 

The  Cup. 

The  Crow. 

The  Centaur, 

The  Wolf. 

The  Altar. 

The  Southern  Crown. 

The  Southern  Fish. 

The  Unicorn. 

Noah's  Dove. 

The  Southern  Cross. 


2 1 8.  In  order  to  learn  the  positions  of  the  various 
constellations  and  stars  you  will  want  a  star-map  or 
planisphere;  and  will  also  require  some  friend  to  point 


ASTRONOMY.  93 


out  to  you  some  of  the  chief  constellations  to  begin 
with.  I  have  indicated  a  few  of  these  by  Roman 
letters  in  the  preceding  lists. 

219.  The  stars  in  each  constellation  are  known  by 
the  prefix  of  some  letter  of  the  Greek  alphabet,  the 
brightest  being  called  Alpha  (a),  the  second  brightest 
Beta  (/?),  and  then,  when  all  the  letters  are  used,  they 
are  numbered  i,  2,  3  ;  so  we  can  refer  to  a  star  as 
Alpha  (a)  Lyrae,  the  brightest  star  in  the  constellation 
of  the  Lyre,  or  (/3)  Cygni,  the  second  brightest  in  the 
Swan,  6 1  Cygni,  and  so  on,  so  that  every  star  can  be 
named.  In  addition  to  these  names  the  principal 
stars  have  other  names,  thus  (a)  Lyrae  is  also  called 
Vega,  a  Canis  Majoris  is  called  Sirius,  a  Bootis,  Arc- 
turus,  and  so  on. 


§  IV. -APPARENT  MOVEMENTS  OF  THE 
STARS. 

220.  We  saw  in  speaking  of  the  earth,  that  it  was 
only  a  moving  observatory,  and  that  therefore  we  must 
distinguish  the  real  motion  of  external  bodies  from 
that  of  the  body  on  which  we  dwell.  We  may  now 
return  to  this  subject.  Let  us  compare  the  earth  to 
a  boat  at  sea;  imagine  yourself  in  the  boat;  then 
if  it  be  suddenly  turned  round,  all  the  ships  in  sight 
will,  if  you  are  ignorant  of  your  motion,  appear  to  go 
round  you  in  the  opposite  direction ;  but  it  would  be 
highly  improbable  that  all  the  ships  in  sight  should 
do  so  at  the  same  rate,  keeping  their  relative  posi- 
tions to  each  other,  so  that  you  would  at  once  find 
out  that  your  boat  was  moving,  and  not  the  ships. 
Just  so,  as  we  have  seen  the  earth  turns  round,  and 


94  SCIENCE  PRIMERS.  [§  v. 

not  the  stars  round  us,  so  the  daily  motion  of  the  stars 
is  only  apparent. 

221.  Now,  let  the  boat  be  rowed  round  a  ship.  The 
relative  positions  of  the  ship  and  the  distant   craft 
change,  the  ship  appears  to  move  round  you,  passing 
between  you  and  the  other  ships  in  succession.     The 
same  appearance  would  be  produced  were  the  boat  to 
remain  still,  and  the  distant  ships  to  move  round  it, 
but  you  would  at  once  detect  that  it  was  your  own 
motion.     Just  so  with  our  annual  revolution  round  the 
sun,   the   sun   apparently   passes   over  the    stars   in 
succession,  the  stars  which  are  in  a  line  with  the  sun 
in  summer  being  opposite  to  him  in  winter. 

222.  In  the  early  days  of  Astronomy  these  two  ap- 
parent motions  of  the  stars  were  the  only  ones  known, 
and  in  order  to  ascertain  whether  the  stars  were  really 
fixed  maps  of  them  were  made,  to  be  compared  with 
the  stars  in  the  course  of  a  few  years,  and  from  the 
comparisons  made  in  this  way  no  alteration  of  position 
was  detected,  so -the  ancients  concluded  that  the  stais 
were  fixed ;  hence  the  term  "  fixed  star,"  but  this  we 
shall  see  was  an  error  caused  by  the  inaccuracy  of 
the  maps. 

223.  When  in  after  years  a  better  method  of  fixing 
the    positions   of   stars    was    invented,    it   was   soon 
found,  that  the  positions  of  the  stars  were  not  always 
the    same,    and    that    this    was    occasioned    by   the 
poles  of  the   earth  changing  the  direction  in  which 
they  pointed,  just  as  a  spinning  top,   before  falling, 
whobbles;    and   so   of   course,  as  the  positions  de- 
pended on  the  position  of  the  earth's  axis  they  were 
found   to  be  continually  changing.      Here,    then,    is 
another  apparent  change  in  the  positions  of  the  slars, 


ASTRONOMY.  95 


and  this  apparent  motion  gives  rise  to  what  is  called 
the  precession  of  the  equinoxes. 

224.  Now  that  astronomers  are  aware  of  this  and 
other  motions,  they  expect  to  find  a  continual  change 
in    the    position   of   stars,  which  they  can   calculate 
beforehand,   but  if   the  positions  of  stars  are  found 
after  a  lapse  of  years    not    to   correspond  with  the 
calculated  ones,  after  allowing  for  all  the  known  ap- 
parent motions,  there  must  be  some  motion  of  the 
earth    or  stars    which   was  not  taken    into   account. 
But,  before  we  go  further,  we  will  return  to  our  boat 
and  ship. 

225.  Let  the  ship,  and  the  boat  you  are  in,  ad- 
vance in  any  direction,  what  apparent  changes  will 
be  produced  in  the   ships   on  either  side  of   you? 
They  will  appear  to  move  in  the  opposite  direction  ; 
those  you  approach  will  appear  to  get  further  apart, 
and  those  behind  you  will  appear  to  close  together,  but 
the  ships  may  all  be  moving  as  well  as  you,  some  in  one 
direction,  and  some  in  another,  so  they  all  may  not 
appear  to  move  regularly  according  to  our  supposition ; 
but  if  there  is  a  large  number  visible,  you  would  expect 
to  find  more  apparently   moving   according   to   our 
supposition  than  contrary  to  it,  their  apparent  motions 
being  counterbalanced  in  some   cases  by  their  real 
motions,  and   in  others  the   two   motions   would  be 
added  to  each  other,  so  that  you  could  judge  of  your 
own  motion. 

226.  This  is   exactly  the  case;    it   is   found   that 
in  one  direction  the  stars  have  a  tendency  to  close 
up,  and   in    the    opposite  one  to  open  out,   though, 
like    the    ships,   some  close  up  in  the    direction  in 
which  the  majority  open  out  and  vice  versa;  but  by 

10 


96  SCIENCE  PRIMERS.  [§  vi. 

observing  the  motion  of  a  large  number  of  stars  we 
are  able  to  find  that  the  sun,  and  with  it  of  course 
all  the  planets,  are  steadily  progressing  towards  a  point 
in  the  constellation  Hercules. 

§  V.— REAL  MOVEMENTS  OF  STARS. 

227.  If  you  saw  any  ship  moving  among  the  others 
whose  motion  was  not  accounted  for  on  the  supposition 
of  any  motion  of  your  boat,  you  would  at  once  pre- 
sume that  that  ship  had  a  real  motion  of  its  own.     In 
like  manner,  when  a  star  is  found  to  move  amongst 
tht  others,  then  we  can  safely  say  it  has  a  real  motion 
of  its  own;  and  by    careful   observation   for  a   long 
series  of  years  it  has  been  discovered  that  a  very  large 
number  of  stars  have  what  is  called  a  proper  motion. 
Arcturus,  for  instance,  is  going  at  about  three  times 
the  rate  that  our  earth  does  in  its  orbit  round  the 
sun,  over  fifty-four  miles  a  second.    From  mechanical 
reasons    it    is    probable   that  all    the   stars   are  in 
motion. 

§  VI.— MULTIPLE  STARS. 

228.  Not  only  have  we  such  a  proper  motion  along 
a  path,  but  some  stars  go  round  each  other. 
These  take  the  name  of  double  and  multiple  stars 
according  as  there  are  two  or  more   moving  round 
each  other,  as  shown  in  Fig.  43. 

229.  They  are  what  is  called  physically  connected 
with   each   other,   being  so  close  that  one  revolves 
round  the  other,  just  as  we  revolve  round  the  sun,  but 
instead  of  the  revolution  being  performed  in  a  year. 


ASTRONOMY.  97 


the  shortest  known  time  of  revolution  or  period  of  a 
double  star  is  thirty-six  years.  Up  to  the  present 
time  some  800  of  these  systems  have  been  discovered. 


FIG.  43. — Orbit  of  a  Double  Star. 

230.  The  distances  of  the  stars  from  us  is  so 
immense  that  if  they  had  planets  revolving  round 
them  these  would  be  invisible  with  our  most  power- 
ful instruments.  But  it  is  probable  that  each  star  is 
the  centre  of  a  planetary  system  :  in  the  case  of  close 
double  stars,  therefore,  the  planets  of  one  star  must  be 
so  near  the  other  as  to  receive  a  considerable  amount 
of  light  from  it ;  in  fact,  the  planets  would  have  two 
suns,  and,  in  some  cases,  suns  giving  light  of  different 
colours. 


§  VII.— CLUSTERS  AND  NEBULA. 

231.  Besides  the  scattered  stars  of  which  we  have 
been  talking,  there  are  a  number  of  white  patches  in 
the  sky  like  little  pieces  of  the  Milky  Way,  a  few  of 
which  are  visible  to  the  naked  eye.  When  these  are 
looked  at  with  a  telescope,  some  of  them  are  seen  to 
be  very  closely  packed  clusters  of  small  stars ;  in  some 


SCIENCE  PRIMERS: 


[§  VII. 


the  separate  stars  are  seen  with  telescopes  of  low 
power,  while  others  require  the  highest  telescopic 
means.  Those  in  which  the  stars  are  easily  seen,  are 
called  clusters,  while  those  requiring  high  powers 
to  see  the  separate  stars,  and  those  which  still  appear 


FIG.  44. — The  Cluster  in  Hercules. 

of  a  cloudlike  structure  when  the  most  powerful 
telescopes  are  brought  to  bear  upon  them,  are  called 
nebulse. 

232.  We  may  therefore  divide  these  objects  into 
three  classes  :  (i)  the  clusters,  in  which  the  separate 
stars  are  easily  seen  gradually  merging  into  (2)  the 
resolvable  nebulae ;  and  (3)  the  irresolvable 


ASTRONOMY. 


99 


nebulae.    The  spectroscope  has  shown  some  of  these 
latter  to  be  of  a  nature  different  from  stars  or  a  col- 


lection of  stars,  and  so  in  this  they  are  unlike  the 
clusters. 


ioo  SCIENCE  PRIMERS.  [§  vm. 

233.  Nor  is  this  all :   not  only  have  we  cloudlike 
masses  which   may  be   broken    up   into    stars,  and 
cloudlike  masses  which  we  know  cannot  consist  of 
true   stars,   but   some  stars,  when  closely  examined, 
seem  to  be  surrounded  by  a  kind  of  fog,  and  these 
we  know  are  not  true  stars.     Such  bodies  are  called 
nebulous  stars. 

234.  Both  the  star  clusters  and  nebulae  may  from 
a  different  point  of  view  be  divided  into  two  other 
classes  :  those  which  are  very  irregular  in  shape,  like 
the  Cluster  and  Nebula  shown  in  Figs.  44  and  45, 
and  those  again  which  approach  more  to  a  globular 
form. 


§  VIII.— THE    NATURE    OF    STARS    AND 
NEBULAE. 

235.  I  have  before  told  you  that  the  stars  are  dis- 
tant suns,  but  you  are  not  to  suppose  that  all  of  them 
are  exactly  like  the  sun ;  indeed,  we  have  evidence  that 
they  are  not.     Among  those  which  are  very  bright, 
some  seem  to  have  more  simple  atmospheres  than  the 
sun ;   that  is,  they  do  not  contain  all  the  elements 
stated  in  Art.  204 ;  and  among  those  stars  which  are 
dimmer,    and    especially  among    those    the    light  of 
which  is  reddish,  the  atmospheres  seem  to  differ  in 
character  fiom  that  of  the  sun,  as  if— mark,  I  only 
say  as  if—  such  stars  were  colder  than  the  sun. 

236.  Although  the  nebulae  appear  to  be  very  dif- 
ferent from  stars,  it  is  possible  that  there  is  a  very 
close  connection   between    them,    for    it    has    been 


ASTRONOMY. 


thought  that  stars  are  formed  by  the  coming  together 
of  the  materials  of  which  the  nebulae  are  composed, 
and  that  the  planets  are  formed  in  the  process. 
Whether  nebulae  are  masses  of  glowing  gas,  or  clouds 
of  stones  clashing  together,  and  thus  giving  rise  to  a 
luminous  appearance,  we  do  not  know,  but  the  latter 
view  is  the  more  probable  one. 

237.  The  idea  to  which  I  have  referred,  which 
connects  nebulas  with  stars  and  planets,  supposes 
that  a  nebula  in  its  first  stage  is  continually  getting 
smaller  and  rounder,  and  that  when  it  has  done  so 
perhaps  sufficiently  to  give  rise  to  the  appearance  of 
a  nebulous  star,  getting  hotter  all  the  time,  it  leaves 
behind  it,  round  its  equator,  as  it  still  contracts,  rings 
of  vapour,  something  like  the  rings  of  Saturn  (Art.  170) 
which  eventually  break  and  form  a  globular  'mass  of 
vapour,  which  at  last  forms  a  planet.  All  the  time  the 
centre  is  getting  more  dense  and  hot,  and  at  last,  the 
rate  of  contraction  still  diminishing,  it  shines  out  like 
a  real  sun,  and  thus  goes  on  giving  light  and  heat 
to  those  masses,  now  become  cool  and  habitable,  to 
which  it  originally  gave  birth.  It  thus  shines,  first, 
as  a  bright  star,  which  afterwards  becomes  dim,  and 
perhaps  red,  before  the  state  of  extinction  is  reached 
to  which  it  must  surely  arrive ;  for,  do  not  forget, 
that  any  one  mass  of  matter  must  in  time  cease  to 
give  out  light  and  heat,  whether  that  mass  of  matter 
be  a  coal  in  a  fire  or  a  star  in  the  heaven. 


102  SCIENCE  PRIMERS.  [§  11. 


VI.— HOW  THE  POSITIONS  OF  THE  HEA- 
VENLY BODIES  ARE  DETERMINED, 
AND  THE  USE  THAT  IS  MADE  OF 
THEM. 

§  I.— RECAPITULATION.— STAR  MAPS. 

238.  I  must   now  approach  a  different  branch  of 
my  subject     We  have  gone  through  the  real  motions 
of  the  earth,  moon,  and  planets,  and  more  recently  of 
the  stars,  and  the  apparent  motions  brought   about 
by  the  real  mdtion  of  the  earth.     We  have  referred 
to  the  nature  qf  nebulae,  suns,  and  planets,  and  have 
thus  got  an  idea  of  the  Earth's  true  place  in  Nature — 
how  it  is  a  cool  body  going'  round  a  cooling  star,  both 
planet  and   star  having  probably  resulted  from    the 
condensation  and  consequent  heating  of  a  nebula. 

239.  I  have  also  given  you  an  idea  of  the  starry 
heavens;    how    the    stars — so-called  fixed — have  all 
been   grouped   into    constellations,    and    lettered    or 
numbered  in  the  order  of  their  brightness ;  and  how 
the  sun  by  day,  and  the  moon  and  planets  by  night, 
are  perpetually  changing  their  places  among  the  stars 
with  the  most  perfect  order  and  regularity. 

240.  I  have  now  to  ask  your  attention  to  the  starry 
vault,  considering  the  stars  merely  as  things  the  posi- 
tions of  which  have  to  be  mapped ;  and  I  want  to 
show   you,  first,  how  positions   are  determined,  and 
then  what  use  we  make  of  them. 

241.  If  you  were  clever  enough,  you  might  be  able 
to  make  a  sketch-map  of  the  positions  of  the  stars, 


ASTRONOMY.  103 


but  for  astronomical  purposes  the  positions  of  the  stars 
must  be  known  with  much  greater  accuracy  than 
could  be  attained  by  such  a  rough  attempt,  and  even  if 
such  maps  were  perfectly  accurate  it  would  be  very 
troublesome  to  have  to  refer  to  a  star  as  being  south, 
of,  or  below,  a  well-known  star,  and  to  the  left, 
or  west,  of  another;  another  method  of  fixing  their 
places  for  reference  has  therefore  been  adopted. 

§  II.— POLAR   DISTANCE. 

242.  We  imagine  the  equator  and  poles  of  our  globe 
extended  outwards  to  the  stars,  just  as  their  shadows 
would  be  cast  by  a  light  at  the  centre  of  the  earth 
on   the   imaginary  hollow   globe  on  which  the  stars 
appear  fixed  (called   the    celestial  sphere).     The 
shadow    of    the    earth's    equator  thus  becomes    the 
celestial  equator,  and  we  measure  north  and  south 
to  it  in  degrees  from  the  shadows  of  the  poles,  calling 
this  distance  polar  distance. 

243.  In  this  way  we  can  say  which  star  or  which 
part  of  the  sky  is  exactly  at  the  pole,  because  it  will 
have  no  motion.     Get  your  orange  and  stick  a  pin  in 
it  at  each  pole ;  if  you  turn  the  orange  round,  the  pin 
will  still  point  to  the  same  place.     This,  then,  will  be 
o°  polar  distance.     Now,  with  a  telescope  furnished 
with  circles,  we  can  find  this  spot  in  the  heavens,  and 
turning  the  telescope   10°  from  this  spot  (which  we 
can  easily  do  by  means  of  the    small  circle  fixed  to 
it,  because  you  have  already  seen  that  all  circles  big 
or  little  are  divided  into  360°,  Art.  126)  we  can  deter- 
mine those   stars  which  have  10°  polar  distance,  then 
20°,   30°,  and  so  on,  till  we  come  to  90°,  which  of 


io4 


SCIENCE  PRIMERS. 


course  marks  the  position  of  the  Celestial  Equator — 
that  is,  the  line  in  the  heavens  which  lies  exactly 
half-way  between  the  north  and  south  poles,  as  the 
terrestrial  equator  does  on  the  earth. 


§  III.— POLAR  DISTANCE  IS  NOT  SUFFICIENT. 

244.  In  this  way,  then,  we  can  determine  the  polar 
distance  of  all  the  stars ;  but  you  will  see  at  once  that 
a  multitude  of  stars  may  have  the  same   polar  dis- 
tance, for  we  can  stick  a  whole  row  of  pins  in  the 
orange,  so  that  all  shall  be  the  same  distance  from 
the  pole  of  the  orange  marked  by  another  pin. 

245.  It  is  necessary,  then,  to  distinguish  these  apart 
somehow.     Do  not  forget  that  the  question  is  to  fix 


0                                    C   6               »                         C 

- 

£ 

- 

M 

i 

FIG.  46. — How  to  define  the  position  ol  anything. 

the  position  of  a  star.  Now,  to  begin  with,  how 
would  you  fix  the  position  of  a  dot  on  a  piece  of 
paper?  Let  us  see.  Take  a  sheet  of  paper  A  BCD, 
Fig.  46,  and  stick  a  pin  in  or  make  a  dot  E  on 
it.  Now  let  us  see  how  we  can  state  its  position : 
divide  the  side  AD  into,  say,  10  equal  parts,  and 
AB  into,  say,  the  same  number;  then  on  joining  EG 


ASTRONOMY.  105 


and  EF,  you  will  see  that  E  is  4^  divisions  from 
the  line  AB  measured  along  AD,  and  is  2\  divisions 
from  AD  measured  along  AB,  so  we  can  fix  the 
position  to  this  point  E  at  once  with  reference  to  the 
edges  of  the  paper.  So  also  if  you  were  asked  to  place 
a  dot  at  7  divisions  from  AB  and  6  from  AD,  you 
would  draw  a  line  HI  from  the  seventh  division  on 
AD  and  another  KL  from  the  sixth  division  on  AB, 
then  the  point  M  where  they  cross  will  be  the  place 
required. 

246.  Now  mark  well  that  it  is  not  enough  to  say 
that  E  is  4^  divisions  from  AB,  because  there  might 
have  been  a  whole  line  of  pins  or  dots  at  that  dis- 
tance   from  AB,   and   that  it  is  not  enough  to  say 
that  E   is  2j  divisions    from  AD,  because  in  like 
manner  there  might  have  been  a  whole  line  of  pins 
or  dots  at  that  distance. 

247.  Mark  well  also  that  the  moment  we  have  two 
sets  of  measures  at  right  angles  (you  have  not  for- 
gotten, I  hope,  what  that  means)  to  each  other,  we 
can  state  the  position  of  a  pin  or  dot  on  our  piece  of 
paper  with  the  greatest  accuracy. 

248.  So  it  is  with  the  stars.     I  have  already  made 
you  acquainted  with  one  set  of  measures,  that  which 
begins  at  the  poles  and  measures  the  distances  of  the 
stars  from  the  poles,  or,  what  comes  to  the  same  thing, 
the  distance  from  the  equator,  because  when  we  know 
the  number  of  degrees  a  star  is  from  the  pole,  the  dif- 
ference between  that  number  and  90°  will  give  us  the 
distance  from  the  equator,  as  of  course  the  equator  is 
90°  from  each  pole.     In  the  next  diagram,  Fig.  47,  I 
have  drawn  the  equator  and  straight  lines  10°  apart 
between  it  and  each  pole. 


io6 


SCIENCE  PRIMERS. 


IV. 


§  IV.— RIGHT    ASCENSION. 

249.  Evidently  therefore,  to  make  our  statement  of 
a  star's  position  complete,  we  want  another  line  at 


FIG.  47. — How  the  positions  of  stars  are  stated. 

right  angles  to  these.  Now  get  your  orange,  and 
stick  a  row  of  pins  in  it  all  round  to  mark  the  equa- 
tor A  B  Fig.  47.  Next,  stick  another  row  of  pins  in 
at  right  angles  to  the  first  row  CD.  This  second  row 
will  take  the  shape  of  a  second  circle  of  pins,  passing 
over  the  poles  of  the  orange,  and  cutting  the  equator 
in  two  opposite  points. 

250.  Now  the  equator,  and  the  row  of  pins 
which  represents  it,  can  only  be  in  one  place  on 
the  orange,  that  is  half-way  between  the  two  poles. 


ASTRONOMY.  107 


But  you  may  make  the  second  circle  wherever  you 
choose,  and  in  fact  you  may  suppose  an  infinite 
number  of  such  circles,  all  of  them  at  right  angles 
to  the  equator,  all  cutting  it  in  two  opposite  points, 
all  passing  through  the  poles;  of  course  we  can; 
imagine  them  i°  or  10°,  or  any  other  number  of  de- 
grees apart;  if  we  imagine  them  to  be  15°  apart,  then 
as  the  heavens  appear  to  revolve  round  the  earth  in 
24  hours,  one  of  these  circles  will  pass  over  a  place 
on  the  earth  every  hour,  because  15°  X  24  —  360°. 

251.  But  we  have  not  yet  got  over  our  difficulties. 
All  these  circles  are  alike ;  we  must  therefore  choose 
one  to  measure  from,  to  represent  the  equator,  as  it 
were.     You  will  perhaps  think  that  the  first  will  be. 
made  to  pass  through  the  brightest  star.     This  is  not 
so  ;  one  of  the  two  points  of   the  celestial  equator 
which  lies  exactly  in  the  plane  of  the  ecliptic  (Art.  67) 
is  chosen.     This  point  is  called  the  first  point  of 
Aries. 

252.  This  being  determined  on,  all  the  astronomerr 
has   to  do  is  first  to  regulate  his  clock  so.  that   the 
stars  shall  appear  to  travel  round  the  earth  in  exactly 
24  hours  ;  to  let  it  show  oh  om  o*,  when  this  imaginary 
circle,  which  passes  through  the  first  point  of  Aries,, 
passes  what  is  called  the  meridian,  that  is  a  fixed 
imaginary  circle  passing  from  north  to  south  overhead, 
and  to  note  the  time  when  each  star  also  passes  it. 
As  each  star,  whatever  be  its  polar  distance,  passes 
this  line,  the  clock,  if  it  goes  correctly,  will  show  its 
distance  in  time  from  the  first  point  of  Aries.     Thus 
we  say  that  the  Right  Ascension  of  the  brightest  star 
(o.)  in  the  Bull  is  4h  28™ ;  of  the  brightest  star  in  the 
Virgin,  13"  i8m,  and  so  on. 

11 


lo8  SCIENCE  PRIMERS.  [§  vi. 


§  V.— RECAPITULATION. 

253.  If  you  have  understood  this  you  will  know 
that  the  place  of  a  star  is  stated  or  defined  : — 

First — By  its  distance  in  degrees  from  the  pole. 
This  is  called  its  polar  distance  ;  from  which  (as 
stated  in  Art.  249)  we  can  easily  determine  its  dis- 
tance from  the  equator,  called  its  declination. 

And  Secondly — By  its  distance  in  time  from  the 
great  circle  which  passes  through  the  first  point  of 
Aries.  This  is  called  its  Right  Ascension. 

254.  The   positions  of  all   stars   have   thus   been 
determined,  and  further,  we  can  calculate  what 
position  among  the  stars  the  sun,  moon,  or 
.any  of  the  planets  will  occupy  at  any  instant 
of  time. 

255.  This  is  one  of  the  most  useful  results  of  As- 
tronomical Science,  for  it  enables  us  to  map  the  sur- 
face of  the  earth,  and  also  enables  the  traveller  in  the 
trackless  waste,  or  the  mariner  out  of  sight  of  land, 
.to  find  out  exactly  where  he  is  on  that  surface. 


§  VI.— THE  LATITUDE  OF  PLACES  ON  THE 
EARTH. 

256.  Let  us  see  then  how  we  can  fix  the  position  of 
any  place  on  the  earth.  If  you  were  asked  to  tell 
anyone  where  a  neighbouring  town  or  village  was, 
you  would  probably  say  so  many  miles  away,  and 
along  a  certain  road,  or  in  a  certain  direction,  say 
S.VV.  of  your  house.  This  answers  very  well  for  short 


ASTRONOMY.  109 


distances,  but  it  would  never  do  to  refer  all  places  to 
this  distance  and  direction  from  your  house,  or  from 
any  other  one  place.  If  the  earth  were  flat  we  could 
use  the  method  referred  to  in  Art.  246,  but  as  the  earth 
is  not  flat,  we  do  this  ;  we  measure  from  the  equator 
towards  the  pole  in  either  hemisphere,  and  if  you 
refer  to  a  globe  you  will  see  that  there  is  a  number 
of  circles  drawn  at  equal  distance  apart  between 
the  poles  and  the  equator.  These  circles  are  called 
parallels  of  latitude. 

257.  Remember,  that  the  positions  of  the  heavenly 
bodies  have  been  determined  with  reference  to  the 
earth's  pole  and  by  means  of  its  rotation.  Now,  if  you 
will  think  a  little,  you  will  see  that  if  there  were  a  star 
known  to  be  of  o°  north  polar  distance,  that  star  would 
be  exactly  over  your  head  if  you  were  at  the  north 
pole,  and  therefore  you  would  know  you  were 
at  the  pole  if  that  star  appeared  fixed  exactly 
over  your  head.  If  there  were  a  star  known  to  be 
of  90°  polar  distance,  that  star  would  be  exactly  over 
your  head  if  you  were  at  the  equator ;  and  therefore 
you  would  know  that  you  were  at  the  equator 
if  that  particular  star  passed  over  your  head. 

258^  Similarly,  for  any  place  north  or  south  of  the 
equator,  we  can  determine  the  distance  in  degrees  of 
that  place  from  the  equator,  by  observing  which  star, 
or  other  heavenly  body  the  declination  (Art  253)  of 
which  is  known,  passes  overhead.  And  this  is  the 
meaning  of  the  equator,  and  of  the  circles  parallel  to 
it,  you  see  in  maps  and  globes.  An  observation,  the 
principle  of  which  I  have  stated,  must  have  been 
made  before  the  positions  of  any  places  were  laid 
down.  Thus,  in  maps,  you  will  find  the  distance  of 


no  SCIENCE  PRIMERS.  [§  vii. 

London  from  the  equator  shown  as  5i|°  N.,  be- 
cause the  star  y  Draconis,  with  a  north  declination 
of  5 1  J°,  passes  exactly  over  London. 

259.  This  distance  from  the  terrestrial  equator  is 
called  latitude,  the  distance  from  the  celestial  equa- 
tor being  called  declination  (it  is  a  pity  that  the  same 
word  is  not  used  for  both),  and  we  have  of  course  N. 
and  S.  latitude,  as  we  have  N.  and  S.  declination. 

260.  The  latitude  of  a  place  can  also  be  determined 
.by  the  apparent  altitude  of  the  pole  star  above  the 
horizon,  just  in  the  same  way  as  the  rotundity  of  the 
earth  is  determined.    The  observer  at  the  equator  sees 
the  north  polar  star  on  his  horizon,  its  altitude  is  then 
o°,  but  if  he  goes  about  68J  miles  north  it  is  i°  above 
his  horizon,  his  latitude  is  said  then  to  be  i°,  and  so 
on,  gradually  increasing  up  to  90°  at  the  poles.     So  if 
we  at  any  place,  or  time,, measure  the  altitude  of  the 
pole  star,  we  at  once  get  our  latitude  and  can  then  fix 
our  position  on  a  map  or  globe. 

261.  We  have  imagined  such  a  pole  star  for  these 
observations  for  the  sake  of  simplicitv,  but  in  reality 
there  is  no  star  absolutely  at  the  pole,  what  is  called 
the  pole  star  being  about  ij°  from  it,  so  that  allow- 
ance has  to  be  made  for  this. 

262.  It  will  be  clear  to  you  that,  for  the  same  reason 
that  a  large  number  of  pins  on  your  orange  can  be  at 
the  same  distance  from  the  pole  of  the  orange,  and  a 
large  number  of  stars  may  have  the  same  polar  dis- 
tance, so  a  large  number  of  places  on  the  earth  may 
have  the  same  latitude.     Thus,  Naples  has  nearly  the 
same  latitude  as  Pekin  and  New  York. 


ASTRONOMY.  m 


§  VIL-THE  LONGITUDE  OF  PLACES  ON  THE 
EARTH. 


263.  To  determine  finally,  then,  the  position  of  a 
place  on  the  earth's  surface,  we  want  something  else 
which  shall  do  for  the  earth  what  right  ascension 
does  for  the  heavens.     This  something  else  is  called 
longitude. 

264.  To  accomplish  this,  geographers  imitate  astro- 
nomers ;  they  imagine  a  circle  belting  the  earth,  cutting 
the  Terrestrial  Equator,  at  right  angles,  at  two  oppo- 
site points,  and  passing  through  the  poles  of  the  earth ; 
and  they  measure  from  this  circle. 

265.  You  will  naturally  ask  where  this  is.     It  really 
does  not  matter  where  this  start-point  is  taken ;  so, 
as  a  matter  of   fact,    each  principal    nation    of   the 
world  uses  a  different  one,  taking  that  which  passes 
along  the  spider  line  which  marks  the  centre  of  one 
of  the  chief  instruments  in  the  Central  Observatory. 
In  England,  for  instance,  we  reckon  from  the  circle 
which  passes  through  the  Greenwich  Transit  Instru- 
ment.   In  America  they  reckon  in  the  same  way  from 
Washington  Observatory;   in  France  from  the  Paris 
Observatory,  and  so  on. 

266.  The  next  question  is,  how  do  they  measure  ? 
The  position  of  a  place  on  the  earth,  east  or  west  of 
the  circle  which  passes  through  the  real  Greenwich, 
is    determined    in  exactly  the  same  manner  as  the 
position  of  a  star  is  determined  east  or  west  of  the 
circle  which  passes  through  the  imaginary  first  point 
of  Aries.     It  is  a  question  of  time. 


112  SCIENCE  PRIMERS.  [§  vii. 

267.  To  prove  .this,  let   us  again  use  the  orange 
and   knitting-needle.      Represent   the  circle   passing 
through  the  poles  and  Greenwich  by  a  row  of  pins. 
Let   each  pin  represent  an  observer  with   a   watch 
showing  the  time  of  the  Greenwich  clock,  and  let 
one  of  them  represent  the  observer  at  Greenwich ; 
let  a  candle  or  lamp  represent  a  star,  and  rotate  the 
orange  from  west  to  east,  as  shown  in  Fig.  9,  to  repre- 
sent the  motion  of  the  earth.     The  line  of  pins  will 
all  come  between  the  candle  and  the  knitting-needle 
at  once.     Therefore,  all  the  watches  of  our  imaginary 
observers  will  note  the  passage  of  the  imaginary  star 
at  the  same  moment. 

268.  So  that  all  places  exactly  north  or  south  of 
Greenwich  will  have  the  same  start-point  of  time  as 
Greenwich  itself ;  in  other  words,  they  will  have  the 
same  longitude. 

269.  Now  take  out  the  pin  representing  Greenwich, 
and  put  it  to  the  west  of  the  row  of  pins.     As  the 
orange  must  still  be  moved  from  west  to  east,  it  is 
clear  that  this  pin  will  come  between  the  lamp  and 
the  knitting-needle  after  the  row  has  passed ;   that 
is,  there  will  be  a  difference  in  the  times  at  which 
the  row  of  pins  and  the  solitary  pin  pass  the  lamp, 
since  all   the  watches  are    set   to    Greenwich  time. 
Let  us  suppose  that  at  the  row  of  pins  the  Greenwich 
time  is  ih  ;  then  it  is  clear,  that  as  the  pin  representing 
Greenwich  passed  under  the  lamp  afterwards,  the  clock 
at  Greenwich  itself  indicated  some  time  after  ib,  let  us 
say  it  was  2*.     Then  there  is  a  time  difference  of  one 
hour  between  the  two  places,  and  all  the  places  of 
the  same  longitude  represented  by  the  row  of  pins 
will  be  shown  to  the  east  of  Greenwich. 


ASTRONOMY.  113 


270.  Now  let  the  lamp  represent  the  sun.    The  sun 
brings  local  time  to  a  place,  because  it  is  12  o'clock 
(near  enough  for  our  present  purpose)  at  a  place  when 
the  sun  is  south  or  crosses  the  meridian  at  midday. 
If  therefore  I    have  this  local  time  and  Greenwich 
time  as  well,  I  can  tell  first  whether  I  am  east  or 
west  of  Greenwich,  and  then  how  far  east  or  west.     If 
when  with  me  it  is  10  A.M.  it  is  12  (noon)  at  Greenwich, 
then  I  am  situated  to  the  west  of  Greenwich,  and  the 
earth  must  turn  for  two  hours  before  I  am  brought 
under  the  sun;  if  it  is  2  P.M.  with  me  when  it  is  12 
(noon)  at  Greenwich,  then  I  am  to  the  east  of  Green- 
wich, as  I  passed  under  the  sun  two  hours  ago.    Such 
a  difference  of  time  of  12  hours  =  180°;  of  6  hours 
=  90°  east  or  west ;  of  3  hours,  45°  east  or  west,  and 
so  on ;  so  that  it  is  immaterial  whether  we  reckon 
longitude  in  degrees  or  hours,  for  since  there  are  360 
degrees  or  24  hours  into  which  the  equator  is  divided, 
each  hour  .corresponds  to  15°.     We  also  express  the 
longitude  of  a  place  by  its  distance  east  of  Green- 
wich in  hours,  so  instead  of  calling  a  place  twenty- 
three  hours  west  it  is  called  one  hour  east. 

271.  In  practice  a  difficulty  arises  in  finding  out  at 
a  distance  from  Greenwich  the  exact  time  at  Green- 
wich.    A  great  number  of  ways  have  been  tried,  in 
order  to  let  it  be  known  at  one  observing  station  what 
time  it  is  at  the  other.     Rockets  have  been  sent  up, 
guns  fired,  fires  lit,  and  all  kinds  of  signals  made  at 
fixed  times  for  this  purpose  ;   but  these,  of  course, 
only  answer  for  short  distances,  so  for  long  ones  care- 
fully-adjusted chronometers  had  to  be  carried  from 
one  station  to  the  other,  to  convey  the  correct  time  ; 
but  now,  when  telegraph  wires  are  laid  from  one  place 


ii4  SCIENCE  PRIMERS.  [§  i. 

to  another,  as  from  England  to  America,  it  is  easy  to 
let  either  station  know  what  time  it  is  at  the  other. 
For  ships  at  sea  chronometers  answer  well  for  a  short 
time,  but  they  are  liable  to  variation. 

272.  There   are    certain   astronomical  phenomena 
whose  instant  of  occurrence   can   be   foretold,   and 
which  occur  so  far  away  from  the  earth  that  they  are 
visible  over  a  great  part  of  its  surface  at  the  same 
moment  of  time ;  these  are  published  in  the  Nautical 
Almanacs,  such  as  the  eclipses  of  Jupiter's  moons,  and 
the  position  of  our   own   moon.     Suppose   that   an 
eclipse  of  one  of  Jupiters  moons  is  to  take  place  at 

1  P.M.  Greenwich  time,  and  it  is  observed  at  a  place 
at  2  P.M.  of  their  local  time,  /".*.,  two  hours  after  the 
sun  had   passed  the   meridian,   then   manifestly  the 
clock  at  Greenwich  is  at   i   P.M.  while  theirs  is  at 

2  P.M  ,  and  the  difference  of  local  time  is  one  hour, 
and  the  place  is  one  hour  or  1 5°  east  of  Greenwich. 
If,  however,  the   eclipse  was  observed  at  12  noon, 
then  the  place  must  be  one  hour  west  of  Greenwich. 

VII.— WHY  THE   MOTIONS   OF    THE    HEA- 
VENLY BODIES  ARE  SO  REGULAR. 

§  I.— WHAT  WEIGHT  IS. 

273.  We  have  just  seen  that  the  stars  are  so  useful 
to  man  because  we  can  exactly  calculate  in  what  part 
of  the  heavens  they  will  be  at  any  future  time.     Now 
of  course  if  their  motion  or  our  motion  were  irregular, 
this  could  not  be  done.     Before  I  complete  my  task 
then  I  must  attempt  to  explain  to  you  how  it  is  that 
we  are  enabled  to  foretell  the  movements. 


ASTRONOMY.  115 


274.  This  brings  us  to  the  more  mechanical  part  of 
Astronomy,  the  laws  of  the  motions  of  the  heavenly 
bodies.     The  ancients  believed  the  earth   to   be   at 
rest  and   the  sun   and  planets  to  revolve   round   it. 
This   idea,  however,    gave  way  for   the   correct  one 
which  has  been  stated,  and  then  came  the  question, 
Why  do  they  so  revolve  ?     It  was  first  supposed  that 
the  planets  were  carried  round  in  a  vortex  or  whirl- 
pool of  some  kind ;  and  it  was  afterwards  shown  that 
the  planets  revolve  round  the   sun   and  the   moons 
round  their  primaries,  not  exactly  in  circles,  but  what 
are  called  ellipses,  having  the  sun  not  quite  in  the 
centre.     Newton  showed  that  on  mechanical  princi- 
ples they  ought  to  do  so,  and  I  must  try  to  show  you 
why. 

275.  You  have  doubtless  often  seen  a  ball  or  stone 
thrown  up  in  the   air   and  fall   again    to   the   earth. 
Did   you   ever  ask  yourself  the  question,  why  does 
it  fall?     Probably  not;   but  if  you  were  asked  you 
would  probably  answer,  "  Because  all  things  that  are 
heavy  fall  to  the  earth  ; "  and  so  you  would  get  out  of 
the  difficulty,  but  only  to  get  into  another.     Why  are 
things  heavy?  is  the  next  question.     The  answer  is, 
that  all  substances  attract  each  other  in  the 
same  manner  as  a  magnet  attracts  iron ;  so 
one  stone  attracts  another  stone,  but  with  very  small 
force,    and   the   earth  being   an   immense    mass    of 
different  substances  attracts  all  things  on  it  with  such 
a  force  that  the  attraction  of  one  stone  on  another 
is  inappreciable  in  comparison. 

276.  The  weight  or  gravity  therefore  of  anything 
only  means  the  force  with  which  the  earth  attracts  it, 
and  causes  it  to  gravitate  towards  itself. 


Ii6  SCIENCE  PRIMERS. 


ii. 


277.  Now  the  attractive  power  of  bodies  is  in  pro- 
portion to  the  amount  of  matter  they  contain.     For 
instance,  if  the  earth  were  doubled  in  size,  still  being 
made  of  the  same  materials,  it  would  attract  every- 
thing on  it  with  double  the  force  it  now  does,  and 
consequently  everything  would  weigh  double  its  pre- 
sent weight — so  that  then  our  legs  would   have   to 
carry  as  much  weight  as  if  there  were  a  person  on 
our  back  continually.     Also  if  we  double  the  quantity 
of  matter  attracted  by  the  earth,  the  force  with  which 
it  is  attracted,  or  its  weight,  is  also  doubled.     For 
instance,  a  pint  of  water  weighs  one  and  a  quarter 
pounds,  two  pints  therefore  weigh  two   and  a   half 
pounds. 

278.  I  have  before  (Art.  135)  made  use  of  the  words 
quantity  of  matter  or  mass.     A  pint  of  lead 
contains  a  greater  quantity  of  matter  or  has  a  greater 
mass  than  a  pint  of  water,  and  the  word  mass  is 
practically  only  another  word  for  weight  so  long  as  we 
are  on  the  earth ;  but  a  pound  weight  here  would 
weigh  over  two  pounds  at  Jupiter,  although  the  quan- 
tity of  matter  or  mass  is  unchanged.     So  in  dealing 
with  the  weights  of  bodies  under  different  attractions 
we  must  use  a  word  expressing  a  constant  quantity  of 
matter. 

279.  If  our  earth  were  doubled  in  size,  a  pound 
weight  would   still   balance   another   pound   weight 
in   the   scales,   for  both   would   have   their   weights 
increased  really  to  two  pounds  ;  so  we  must  use  some 
other  means  to  determine  any  alteration  of  the  force 
of  gravity. 

280.  A  spring  can  be  arranged  so  as  to  answer  the 
purpose,  as  it  is  not  altered  in  any  way  by  gravity;  but 


ASTRONOMY.  117 


the  most  accurate  method  is  to  ascertain  the 
distance  through  which  a  body  falls  to  the  earth  in  a 
certain  time,  usually  one  second,  since  the  greater  the 
attraction  the  quicker  will  be  the  fall ;  at  the  surface 
of  the  earth  a  body  will  fall,  in  a  vacuum  or  space 
without  air  to  resist  it,  16  feet  in  one  second,  and  at 
the  end  of  that  second  its  velocity  is  32  feet  a  second, 
— that  is,  if  the  force  of  gravity  ceased  at  the  end 
of  the  second  it  would  go  on  through  32  feet  in  the 
next  second. 

281.  The  force  of  gravity  at  the  earth's  surface  is 
therefore  represented  by  32.    On  the  surface  of  Jupiter 
the  force   of  gravity  is  2\  times  that  of  our    earth 
and  is  represented  by  78,  meaning  that  in  one  second 
a  body  allowed  freely  to  full  would  attain  a  velocity 
of  78  feet  a  second. 

§  II.— GRAVITY  DECREASES  WITH  DISTANCE. 

282.  I  have  already  told  you  that  the  weight  of 
anything  on  the  earth  means  the  force  with  which  the 
earth  attracts  it.     I  have  now  to  add  that  this  force 
is  not  the  same  for  bodies  at  different  distances  from 
the  earth. 

283.  Those  of  you  who  have  had  a  magnet  in  your 
hands  have  probably  noticed  that  pieces  of  iron  are 
attracted  the  more  strongly  the  nearer  they  are  to  the 
magnet ;  this  is  easily  seen  by  laying  a  needle  on  the 
table  and  sliding  a  magnet  towards  it,  when  you  will 
see  that  at  a  distance  of  a  few  inches  the  needle  is 
not  attracted  with  sufficient  force   to  overcome  the 
friction  of  its  rolling  on  the  table,  and  the  magnet 


1 1 8  SCIENCE  PRIMERS.  [§  1 1 1 . 

has  to  be  moved  nearer  to  it  until  the  force  is  suf- 
ficient to  overcome  the  resistance,  when  the  needle 
rushes  to  the  magnet. 

284.  It  is  just  the  same  with  gravitation,  the  further 
a  body  is  away  from  the  earth  the  less  it  is  attracted; 
and  Newton  found  that  the  force  of  gravity  at  double 
the  distance  was  not  half,  but  half  of  a  half,  or  one 
quarter;  at  three  times  not  a  third,  but  a  third  of  a 
third,  or  one  ninth,  and  so  on ;  so  if  the  distance  be 
increased  to  eight  times,  we  have  to  multiply  eight  by 
itself,  or  what  is  called  square  it,  making  64,  and 
placing  i  over  it,  making  -^  showing  that  the  attraction 
at  eight  times  the  distance  is  only  one  sixty-fourth  of 
what  it  was  originally. 


§  III.— HOW  THIS  EXPLAINS  THE  MOON'S 
PATH  ROUND  THE  EARTH. 

285.  Newton  tested  this  by  the  motion  of  the  moon 
in  the  following  manner  :    The  moon,  as  we   have 
already  found,  revolves  round  the  earth ;  but  we  have 
not  seen  yet  why  it  should  do  so.     Now,  however,  we 
are  prepared  to  rind  that  it  is  held  in  its  nearly  circular 
orbit  by  the  attraction  of  the  earth  acting  on  it  as  a 
sling  does  on  the  stone,  preventing  it  from  flying  off, 
as  it  would  do  if  the  string  of  gravity  were  cut,  just  as 
the  stone  flies  away  in  a  straight  line  when  the  sling 
is  released. 

286.  Let   us   consider  this  with  the  help   of    the 
diagram,  where  E  represents  the  earth  and  MB  A  the 
orbit  of  the  moon ;  and  let  us  suppose  the  moon  to 
be  at  M;  then  if  gravity  ceased  to  act,  the  moon  would 
continue  on  in  the  same  straight  line  that  it  was  moving 


ASTRONOMY. 


119 


in  at  the  time  gravity  ceased  to  act,  and  would  go  on 
towards  N;  and  in  one  second  it  would  get,  say  to  M\ 
but  by  the  action  of  gravity  we  find  the  moon  actually 
at  B,  showing  that  the  earth's  attraction  has  had  the 
effect  of  drawing  it  from  M'  to  B,  and  since  we  know 
the  dimension  of  the  moon's  orbit,  it  is  only  a  matter 
of  calculation  to  find  the  distance  from  M'  to  B  through 
which  the  earth  draws  the  moon  in  one  second,  which 
is  a  little  under  one-eighteenth  of  an  inch. 


1  IG.  48.— The  fall  of  the  Moon  towards  the  Earth. 

287.  Let  us  see  if  this  fact  falls  in  with  Newton's 
idea.  What  distance  ought  a  body  to  fall,  or  be  at- 
tracted through  in  one  second,  at  the  distance  of  the 
moon?  The  moon  is  240,000  miles  from  the  earth 
roughly,  and  the  surface  of  the  earth  is  4,000  miles 
from  its  centre,  at  which  point  we  can  consider  the 
whole  attraction  concentrated,  and  4,000  into  240,000 
goes  sixty  times,  so  that  the  moon  is  just  sixty  times 
further  from  the  earth's  centre  than  the  surface  is  ;  and 
the  attraction  there  should  be  sixty  times  sixty,  or 
3,600  times  less  at  the  moon's  distance;  but  the  force 
of  gravity  at  the  surface  of  the  earth  is  such  that 

19 


120  SCIENCE  PRIMERS.  [§  iv. 


bodies  fall  sixteen  feet  a  second,  so  at  the  distance 
of  the  moon  they  should  fall  •y-faf  of  sixteen  feet, 
or  one-eighteenth  of  an  inch,  which  as  we  have  seen 
is  the  observed  amount. 


§  IV. -THE  ATTRACTION  OF  GRAVITATION. 

288.  In  this  way  Newton  discovered  that  the  very 
same  force  that  draws  a  stone  to  the  earth,  called 
the  attraction  of  gravitation,  keeps  the  moon  in  her 
path  round  the  earth.  Nor  did  the  discovery  end 
here,  he  showed  that  the  earth  and  all  the  other  planets 
were  thus  kept  in  their  orbits  round  the  sun  ;  and  that 
the  same  law  of  gravitation  holds  good  with  the  most 
distant  star.  All  the  apparently  irregular  motions 
of  the  heavenly  bodies  have  been  reduced  to  law  and 
order  by  Newton,  who  showed  that  all  the  motions  were 
really  regular,  and  therefore  could  be  calculated  before- 
hand. He  thus  enabled  mankind  not  only  to  admire 
the  divine  beauty  and  harmony  of  the  universe  in 
which  we  dwell,  but  to  make  use  of  the  motions  of 
the  heavenly  bodies  for  purposes  of  daily  life. 


THE   END. 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

LOAN  DEPT. 

This  book  is  due  on  the  last  date  stamped  below,  or 

on  the  date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 


24 


1967  5g 


MM  2  7  '67  -5  P 


LOAM 


JUN 


RECEW 


tu 


l    T  iKrat-17 


NICHOLSON'S    GEOLOGY. 

«  Tfxt-Book  of  Geology i  for  Schools  and  Colleges. 

By  H.  ALLEYNE  NICHOLSON,  M.  D.,  D.  Sc.,  M.  A.,   PH.  D., 

F.  R.  S.  E.,  F.  G.  S.,  etc.,  Professor  of  Natural  History 

and  Botany  in  University  College,  Toronto. 

iimo.     266  Pages.    Price,  $1.50. 

This  work  is  thoroughly  adapted  for  the  use  of  beginners.  At  the  £ame 
time  the  subject  is  treated  with  such  fulness  as  to  render  the  work  suitable  foi 
advanced  classes,  while  it  is  intended  to  serve  as  an  introduction  to  a  Larger 
work  which  is  in  course  of  preparation  by  the  author. 

NICHOLSON'S    ZOOLOGY. 

Text-Book  of  Zoology,  foi  Schools  and  Colleges. 
BY  SAME  AUTHOR  AS  ABOVE. 

tzmo.    3S3Jagfs.    Price,  $1.75. 

In  this  volume  much  more  space  has  been  devoted,  comparatively  speaking, 
to  the  Invertebrate  Animals,  than  has  usually  been  the  case  in  works  of  thii 
nature;  upon  the  belief  that  all  teachings  of  Zoology  should,  where  possible, 
be  accompanied  by  practical  work,  while  the  young  student  is  much  more 
likely  to  busy  himself  practically  with  shells,  insects,  corals,  and  the  like,  than 
with  the  larger  and  less  attainable  Vertebrate  Animals. 

Considerable  space  has  been  devoted  to  the  discussion  of  the  principles  CM 
Zoological  classification,  and  the  body  of  the  work  is  prefaced  by  a  synoptical 
view  of  the  chief  divisions  of  the  animal  kingdom. 

*«*  A  copy  of  any  of  the  above  works,  for  examination,  will  be  sent  by  mail, 
post-paid,  to  any  Teacher  or  School-Officer  remitting  one-half  its  price. 

D.  APPLETON  &  CO.,  PUBLISHERS, 

549  &  SS1  BROADWAY,  NEW  YOUL 


QUACKENBOS'S 

NATURAL     PHILOSOPHY. 

A  NATURAL  PHILOSOPHY:  embracing  the  most  recent  Discoveries  in  the 
various  Branches  of  Physics,  and  exhibiting  the  Application  of  Scientific 
Principles  in  Every-day  Life.  Adapted  to  use  with  or  without  Apparatus,  and 
accompanied  with  Practical  Exercises  and  Numerous  Illustrations.  By  G.  P 
QUACKKNBOS,  LL.  D.  Revised  edition  (1871).  izmo.  450  pages.  $1.75. 


CORNELL'S   PHYSICAL  GEOGRAPHY. 

A  PHYSICAL  GEOGRAPHY:  accompanied  with  Nineteen  Pages  of  Maps,  a 
great  Variety  of  Map-questions,  and  One  Hundred  and  Thirty  Diagrams  and 
Pictorial  Illustrations ;  and  embracing  a  detailed  Description  of  the  Physical 
Features  of  the  United  States.  By  S.  S.  CORNELL,  Large  410.  104  pages 
$1.60. 


HUXLEY  &   YOUMANS'S   PHYSIOLOGY. 

THB  ELEMENTS  OF  PHYSIOLOGY  AND  HYGIENE.  A  Text-book  for  Educa- 
tional Institutions.  By  THOMAS  H.  HUXLEY,  F.  R.  S.,  and  WILLIAM  JAY 
YOUMANS,  M.  D.  i2mo.  420  pages.  $1.75. 

Prof.  Huxley  ranks  among  the  first  of  living  physiologists,  and  his  opinions 
are  received  with  deference  by  the  most  advanced  minds.  This  book  was 
written  by  him  for  ihe  purpose  of  clearing  the  subject  from  the  crude  state* 
ments  and  doubtful  doctrines  which  had  crept  into  the  popular  text-books 
through  the  incompetence  of  compilers. 

The  general  subject  of  Hygiene,  prepared  by  Dr.  Youmans,  b  treated  In  a 
series  of  chapters,  bearing  the  following  titles :  The  Scope  and  Aim  of  Hygiene; 
Air  and  Health;  Water  and  Health;  Food  and  Health;  Clothing  and  Health; 
Exercise  and  Health ;  Mental  Hygiene. 

D.  APPLETON  &  CO.,  PUBLISHERS, 

549  A  551  BROADWAY,  NEW  YOUL 


PRIMERS. 


INTRODUCTORY  :   PROW  SSOR  HUXLEY,  F.  R,  S. 
CHEMISTRY;  PROFESSOR  ROSCOB,.  *...&.& 
PHYSICS  :  PROFESSOR  BALFOUR  STEWART,  F,  R,  S. 
PHYSICAL  GEOGRAPHY  :  PROFESSOR  A,  GKI 

F,  R,  S. 

GEOLOGY":   PROFRSS^R  A,  GF.IKTK,  F,  R,  S, 
PHYSIOLOGY:  I>K.  M-  FOSTKK?  F,  R.  S, 
ASTRONOMY;  J.  NOPMAN  LOCKYER,  F.  K.  S. 
BOTANY"'   DR.  J.  D.  HOOKFF,,  CB-:,  F.  K.  S, 


EUROPE;  E.  A.  FREEMAN,  B,  C,  L.,  LL, 
ENGLAND:  J.  R.  GREEN,  M,  A, 
GREECE;  C.  A.  FYFFE,  M,A,. 

HOME?    M.    CREIGHTQNfM.  A, 

FRANCE^  CHARLOTTE  M.  VONGE, 
GEOGRAPHY  :  GEORGE  GROYE,  -ESQ, 


ENGLISH  GRAMMAR:   DR,  R,  MORRTS, 
ENGLISH      LITERATURE:       REV,     STOPFORP 

LATIN   LITERATURE;   REV,  DR.  F,  W,  FARRAR, 
PHILOLOGY":  J.  jPsiuSj,  M,A, 
GREEK  LITERATURE?  K.  C  JEBB,  M,  A, 
THE  BIBLE  :  GEORGE  G  ROVE,  ESQ, 


